I have coached runners and marathoners for many years and custom-designed many training programs.  I've also completed 97 marathons along the way.  Here's my 2¢ worth.

A. Plan ahead and make a commitment
    Hal Higdon wrote "The difference between a Runner and a Racer is an entry form."  Pick out the race you want to run, and then register and train for it.
    If you plan to run the Marine Corps Marathon in October, I suggest you select a 10-miler or half-marathon in September-October.  Some area clubs have low-key 10-milers, or you can run Annapolis 10 Miler in the heat of August with 3000 runners, or Army 10-Miler in October with 16,000.  If you feel that a crowd will push you along, Annapolis is lots of fun, and Army will help you be all that you can be.

B. Rehearse
    Pick out at least one race of 5 miles or 10 kilometers, three weeks before your "big race."  Run it under complete control, not faster than you would run the 10-miler, to build confidence in your ability to cover the distance, and to strengthen your legs.

C. Get help
    Join a club such as DC Road Runners Club or Potomac Valley Track Club.  Learn about DC-area groups on the web page http://pvtc.org/runschedules.html.  On that website, we list 75 free, open group runs in the DC area.  Pick a group near you.  When I'm downtown on Tuesdays or Thursdays, I run with Potomac Runners at 6:30 pm west of the US Capitol. When I'm in Virginia on Thursdays, I run with NOVA at Bluement Park at 6:30 pm.  Every Wednesday, I run with Potomac Runners and DC Road Runners Club at Washington-Lee High School Track in Arlington.
    These clubs welcome you to join at any of these workouts.  Montgomery County Road Runners Club, Annapolis Striders, and Howard County Striders also conduct active, supportive training groups.  These group runs are the mainstay of many runners' schedules in the DC area.

D. Design your training program.
    Understand that concept of periodization.  Visit http://pvtc.org/periodization.html or go to http://TrackCoach.com and find the article on Periodization.  Kevin Jermyn identifies three distinct phases of training:
o  Base building        o  Precompetition        o  Competition
    Operationally, I think Kevin's analysis is right on target.  Many other articles on TrachCoach.Com will also keep you running healthy.
    Conceptually, as I design training programs, I draw from world class coaches Arthur Lydiard, Percy Cerutti, and Bob Allison.  I divide training programs - usually six-month plans - into four periods:
o  Lengthening        o  Strengthening        o  Hardening            o  Sharpening

1. At the beginning of the Lengthening Period, determine your optimum daily and weekly distance.  For 10 miles, your weekly long run should be at least 8 miles, or about 90 minutes.  For the marathon, the long run should be 20 miles, 150-200 minutes.
    Physiologist Hans Selye showed that the body adapts to its maximum stress, rather than to its average stress.  Thus the weekly long run is the focus of the lengthening period.  Do the math.  Since your body stores enough energy to complete 3 times your daily average distance,
o  For 10 miles, your average daily mileage should be 4 miles
o  For the marathon, your average daily mileage should be 9 miles.
    For a 10-miler, and your average is to be 4 miles, your weekly total is 28; subtract 10 for the long run, divide 18 by 6 days, and you can run 3 miles a day, 10 on a weekend day, and finish a 10-miler.
    For a marathon, 9 miles average x 7 days a week = 63 miles a week; 63 miles a week - 20 on the long day = 43 on the other 6 days; and 43 / 6 = 7 miles a day on the other 6 days.  For instance, Jeff Galloway's program uses that principle for a guaranteed-finish marathon; short distance during the week; long distance on the weekend.  If you have already run a marathon, don't start the lengthening period until you are fully recovered -- no blisters, tendinitis, muscle soreness, no lingering illness.  During the Lengthening Period, you build your mileage to its maximum.  To do that, you start by lengthening your long run.  Assume your longest run currently is 4 miles.  This week, make it 5 or 6, but keep your daily runs to 3.  Next week, make one 3 a 4, and make the 6 a 7, but keep the others to 3.  The week after that, make the 4 a 5, make the 7 an 8, make one of the 3s a 4, and so on, until your mileage reaches its maximum.  A reasonable goal is to increase ytour mileage 10% a week - 25 to 28, 28 to 31, 31 to 34, etc.
    At the end of the lengthening period, your mileage should be as much as it can be.  Of course, you must balance your running time with everything else important in your life.  You must balance the five F's:
o  Fitness (your workouts)        o  Fortune (your work)        o  Family & friends, and            o  Faith (your spiritual life)
    You must also learn to eat wisely, drink enough, and get adequate rest.  As you progress in the Lengthening Period, you will notice your body starting to change.  It you are overweight, you will lose a few pounds. If your weight is already ideal, you will notice you can eat more while maintaining weight.  Be sure your fuel is all "high-test" -- fruits and vegetables, low-fat protein (beans, poultry, fish, nutritional yeast), and high fiber carbohydrates.  Low-fiber carbohydrates such as white bread and sugar do not provide long-term energy for training; high-fiber carbohydrates such as whole-wheat bread, brown rice, oats, and corn stay with you longer and provide longer-term energy.
    The goal of this period is to increase your body's ability to store and use fuel, your oxygen uptake capacity, and your blood volume.  Technically, you are increasing the number of mitochondria (energy-using organelles) and red blood cells and building additional peripheral capillary circulation.
    You can take as long as you want in the Lengthening Period. Elijah Lagat, the winner of this year's Boston Marathon, took six years to build up his mileage, going from 160 to 120 pounds in the process.  Keep the workouts easy as you increase your distance.  If you are starting training now for Marine Corps Marathon or a fall
10-miler, you have about 6 weeks for the Lengthening Period, from now through the end of June.  Be careful, however, since hot weather is now upon us.  Learn where all the brinking fountains are on your favorite courses.  Learn the shady courses, away from traffic -- Mount Vernon trail, Theodore Roosevelt Island, W&OD trail, C&O Canal Towpath, Glover Archbold and Battery Kemble Parks, upper Rock Creek Park, Fort DuPont, etc.  In the Washington area, our natural beauty is breathtaking and lifegiving.

2. In the Strengthening Period, put effort into your runs.  Keep your distance just as it was during the Lengthening Period, but add stress.  Add an occasional 5K or 10K race, as a hard, social workout.  Add hills.  Find an uphill point-to-point course, such as Rock Creek Park to East-West Highway or Seven Locks Road up to Democracy Blvd.  Arthur Lydiard popularized a stride technique for the Strengthening Period called Hill Springing -- bouncing from the toes to gain strength uphill.  Once a week, take a time trial over a long distance such as 6 miles.  Learn that course well, and learn what you can do on that course if you try, but don't use your stopwatch for each and every run.  Instead, use it wisely for your time trial.  If you are able to get to the track once a week, do so, but run only run long repetitions, such as 3 x 2 miles or 2 x 3 miles.

    Add weight-lifting two or three times a week.  There are three separate types of weightlifting workouts:
o  Anabolic workouts (Monday, for instance) include 3 sets of 10 repeats you can do comfortably.
o  Catabolic workouts (Wednesday, for instance) include just 3 or 5 repeats of the most weight you can lift.
o  Metabolic (toning) workouts (Friday, for instance, if your long run is Saturday or Sunday) include 20 repeats of light weights.
    You can take as long as you want in the Strengthening Period.  Once you are a strong runner, you can tackle races without ever moving to the next two periods. If you are start training in summer for Marine Corps Marathon or a fall 10-miler, you have about 8 weeks for the Strengthening Period, from early July to late August.  Good races during this period are Rockville Twilighter, DCRRC's Bunion Derby series, and Annapolis 10-Miler.  If you run them, however, use them as hard workouts -- time trials.  Be sure to continue your long runs.
    Your body will change again during the Strengthening Period.  The goal of this period is to increase the strength of your muscles.  Technically, you are increasing both the number and size of your muscle fibers.  Especially if you lift weights, you will see muscle definition and subcutaneous veins and tendons.  Use the visibility of your circulatory system as a guide to hydration.  If you run a long hard workout and then can see your veins more than usual, you are probably dehydrated.  Replenish your water supply.

3. In the Hardening Period, the goal is to teach your muscle fibers to perform when you want them to.  You can cut back your distance somewhat, in favor of harder workouts that leave you tired.  You will need an easy day after your hard workout day.  A typical hard workout in the Hardening Period would be 1-mile repeats.  You might start with 3 the first week; then 3 again, but faster the next week; then 4; then 4 again but faster; then 5; then 5 again, but faster.  You should keep up your weekly long run, increasing its effort each week.  If your chosen "Big Race" is hilly, then at least one of your hard workouts shoud be on a hilly course.
    In this period, you will see marked changes in your body.  Your muscles will quite literally become harder.  In 1980, Toshihiko Seko was second behind Bill Rodgers in the Boston Marathon.  In 1981, Seko's coach confidently predicted victory at Boston.  Reporters asked him "Why?"  He answered, "Because Seko's muscles are as hard as rocks."
    Your Hardening Period can last 4 to 6 weeks.  If you plan Army 10 Miler or Marine Corps Marathon, September is the month for the Hardening Period.  In the Hardening Period, each workout has a definable purpose.  One workout might be distance, the next day recovery, the next day hard hills, the next day recovery, the next day long reps on the track, followed by two easier days before your next long run.  You can choose to stay forever in the Hardening Period.  Once you are a fit athlete, you need not undertake the risk of the next period.  If your goal is a memorable performance in a 10-mile or marathon race, however, or, for that matter, a shorter race, you may wish to go on.

4. In the Sharpening Period, you begin to prepare specifically for the "Big Race."  You should cut back your mileage, except you should continue your weekly or twice-weekly long run.  Your hard days become very hard, followed by easy days, a few miles of what others call jogging.  A typical hard day in the Sharpening Period might be 6 or 8 repeats of 800 meters (a half-mile) for a 10 miler, 10 repeats for a marathoner.
    In the Sharpening Period, even more than the Hardening Period, each workout has a very specific purpose.  One workout might be a time trial at a distance shorter than your competition distance, the next day recovery, the next day long repeats on the track, the next day recovery, the next day short repeats on the track, then two easy days before your next long run.  Don't do hill repeats during the Sharpening Period, unless your "Big Race" is hilly -- then one of your weekly runs should be hilly.  In the two weeks before your "Big Race," begin to taper your training, substituting sleep for training time.  As you taper, be sure also to eat less.  As my friend Jim Hagan said, "You can't eat like a 20-mile-a-day runner if you're running like a 5-mile-a-day eater."
    Listen to your body during the Sharpening Period.  You should run your short-distance reps faster than ever before, a higher level of risk.  If something hurts, let it heal right away by taking an easy day or two.  Learn self-massage.  Learn about heel lifts for Achilles tendinitis; arch supports for plantar fasciitis (1999's Injury-of-the-Year); forefoot shoe padding for knee pain; and side padding for hamstring or iliotibial band syndromes.  I know these all from painful personal experience.  Another quote from Hal Higdon:  "How fast you are depending on how lucky you were born.  How fast you become depends on how good a physiologist you can be."
    The Sharpening Period can last only 4 weeks.  Because it involves more easy days than the first three periods, you will de-train (lose fitness) if you try to extend the Sharpening Period.  The strain you place on your muscles, tendons, and ligaments means a high risk of injury; it is best to minimize that risk by keeping the final period short.  Moreover, your body can stand the fatigue products of very hard training only so long before your resistance wears down and you catch a cold or worse.
    If you are training for Army 10 Miler or Marine Corps Marathon, you should start sharpening at the beginning of October.  It's still light at 6:30 pm, so you can hit the track Tuesdays with NOVA or Wednesdays with DCRRC.  If you are running a fall marathon, DCRRC's National Capital 20-miler in late September can be your last hard 20-mile run.  Then after several easy days, you may start the Sharpening Period.  The weekend before the "Big Race," take one last hard run, but not the length of the competition.  For a 10-miler, 7 or 8 miles would be plenty; for a marathon, 12 or 15.  Take two days of easy running, then at mid-week, a short, very brisk track workout, such as 10 repeats of 400 meters.  Then take the last THREE days before the "Big Race" easy, substituting sleep for training.  If you like pre-race expos, walk the expo on Friday before a Sunday race, rather than pounding your legs with a long walk Saturday before your race.  You haven't come this far and trained this long just to waste it on the last day.  Get plenty of sleep the two nights before the race, prepare your bag the night before instead of the morning of the race, don't eat anything unusual that might upset your stomach, and try both to relax about the upcoming event and avoid distractions.
    I once had to collaborate on a cost proposal via long-distance conference call on the morning of the Boston Marathon.  I did not run well that day.  Worse yet, we did not win the contract.

5. The Competition
    When you get to the race, run your pace, not faster.  Been there, done that too.  At Boston Marathon 2000, I led all 50+ runners at the 5K mark in 18:07.  Another 50+ runner was 20 seconds behind me.  We ended up 21st and 20th in our age group.  That was not wise pacing.
    If your workouts tell you can run 8-minutes miles for 3+ hours, you should start at an 8-minute-per-mile pace and methodically run a 3:30 marathon.  If you are breathless, if you can't keep up with the runners passing you, or if everyone around you speaking Russian or Swahili, you have gone out too fast.  On the other hand, if you feel comfortable, or you're alongside your training buddy, you're at the right pace.  Enjoy the crowds and the experience.  If the day is warm, adjust your expectations and be sure to take water and splash yourself.  If it's cold or windy, conserve your energy, since you'll need it in the last part of the race.  Everyone's goal should be to finish smiling.  After a race, I always quote my boyhood hero, Chicago Cubs shortstop Ernie Banks:  "Let's do it again."
    Plan to meet your mates post-race and socialize.  NOVA, Washington Running Club, Potomac Runners, and other clubs all have had post-race parties in the past.  I love those parties; they reward you emotionally for your physical effort.  After your "Big Race," take a hot bath, show your medal to your friends and family, use anti-inflammatories if you must to help you heal, take days or weeks off to let yourself recover ... and then begin another Lengthening Period.