Note added: People keep finding this by doing a Google search. For good info on training for 100 miles, check out my posts "A Sprinter at an Ultramarathon #1,2,3,4" A good proven plan for beginners is available at http://www.trailrunevents.com/ul/schedule-100m.asp.
When I first decided I was going to run a 100 mile race, I looked in vain for a training schedule. There was a lot of advice, mostly conflicting, and the pay-off was generally: you have to find what works for you. This left me looking at what others did and what's published pertains mostly to world-class runners; it's like trying to find out how to finish a marathon by studying guys who ran 2:30 the first time they tried. Now that I've finished (just barely) two 100's, suffering every injury and indignity on the way, I think I can supply the needed schedule, though this is NOT how I did it - I as a runner do not listen to me as a coach.
24 weeks to 100 miles
Mondays: 105 minutes trail running
Tuesdays: 105 minutes running hills (1/4-1/2 mile long). After a warm-up, 20 minutes up and down at half-marathon pace, then 6 times up hard (5K pace) with easy downhills, then finish out the time easy up (powerwalking) and down.
Wednesdays, Thursdays and Fridays: 30 minutes easy running
Weekends (8 week cycle - trails preferred, except where noted):
1) Saturday: 255 minutes with 75 minutes at marathon pace, Sunday: 105 minutes
2) Saturday: 420 minutes with 75 minutes at marathon pace, Sunday:105 minutes
3) Saturday: 255 minutes (night run on trail), Sunday 105 minutes
4) Saturday: 5-6 hours; Sunday: 3-4 hours
5) Saturday: 255 minutes with 75 minutes at marathon pace on road, Sunday 105 min.
6) Same as week 2.
7) Same as week 1.
8) Saturday: RACE; Sunday 30 minutes
The races on week 8) should be 50 miles, then 50 miles, then 100 miles
Most successful 100 mile runners do very high mileage. The more experience one has in doing long runs, the less important high mileage becomes. This schedule has one doing high mileage for four days, then three easy days each week. The continuous wear of running long (sometimes erroneously referred to as "training the endocrine system") teaches one to run when already tired. Doing it every day is draining and this method gives one a feel for it, while allowing one's body to rebuild.
It may take one months of building one's mileage to be able to start this training schedule. For a 3:00 marathoner, this schedule is about 90 miles per week. For a 5:00 marathoner, it's still about 60 miles per week.
There are runners who get by on much less mileage. Many of them do a lot of cross-training, some just don't seem to need it. This schedule is for those who want to be assured of finishing.
The sadists (er, i mean race directors) who create 100 mile race courses seem to find the hilliest and most difficult terrain on which to run. One of the common problems of those who fail at 100 miles is "dead quads," a condition where one can no longer run downhill. This requires one to train on hills. Running regularly on trails helps, but specific hill workouts are a good idea. The hill workout I give is a bit of a grab bag; one should learn how to run at a steady rate, how to hike uphill and how to run downhill when exhausted.
Long runs and races
"Training for 100 miles is just like training for a marathon, but with longer long runs." This old saying is true, up to a point. Most people can learn to run up to 5 or 6 hours without much difficulty, but there comes a point where one involuntarily slows after running out of energy. Long runs teach one how to get past that point. If one's going to have problems with nausea, hand swelling, chafing, blisters or black toenails (or all of them at once!), it will occur in a 50 mile trail run, though not as badly as in a 100, and one can discover how to remedy the problem or to persevere despite it. Running longer than 50 miles starts becoming counter-productive.
Finding 50 mile races that fit one's schedule is not easy. One can substitute a time-trial, running 50 by oneself, but it is easier to do it with others.
Unlike shorter races, one will have to run at night during a 100 mile trail run (there are a few runners fast enough that they don't - if you're reading this, you're not one of them). Running at night not only tests one's lighting equipment, but teaches one to run after a long day, when one wants to sleep and when one starts making mental errors. It's a good idea to do these on trails, as terrain becomes trickier in the dark. Some runners are accustomed to running only at one time of day; this forces one to run at a different time, perhaps after a meal, which is difficult for some.
I have two long runs on Saturday and Sunday of the 4th week. This combines the grind of high mileage, the difficulty of a long run and running when already tired. If one's going to be racing more than 24 hours, this approximates the start and finish of the race.
I include one long run on roads. This is a different kind of stress than running on trails and one one should learn to handle. It also keeps one honest in appraising one's abilities; it's easy for a trail runner to lose track of what condition he or she is in, as trails can't be compared to each other very well. An occasional run on a road is a reproducible way to keep track of fitness.
Marathon Pace Running
One of the things I found when attempting to do high mileage was that it became difficult to maintain a hard pace for a long time. I include marathon pace running to remedy this problem. By including it in the long runs, one learns to run slowly after one has depleted energy stores.
This schedule does not include a taper before the 100 Mile race, because I think it's important when trying to finish a first 100 to change as little as possible from one's routine. If one wants to taper before the goal race, cutting total mileage the last three weeks by one-third to one-half is reasonable, as is eliminating the last long run before the race.