Prerequisites:
This training program is for those attempting a 100 mile ultramarathon for the first time and starts out with long runs in the 16 to 18 miles range. A 100 mile race about 24 weeks out. If your current long run is more than 18 miles, simply start the training program at the point that matches your current long run and find a race at the appropriate time.
Race Terrain:
The number one rule of training for a 100 mile ultramarathon is to train in similar conditions, on similar terrain, as the rce will be run. If that is not possible, it is going to be very difficult to train appropriately for the race.
Weekly Mileage:
Weekly mileage among ultra-runners also varies dramatically. Those capable of 100+ mile weeks run them, but most ultra-runners cannot handle that much weekly running without causing injuries. A peak training week for a 100 mile race might include 3 days of back-to-back runs of 15 miles, 25 miles, and 30 miles for 70 miles run in those 3 days. All prior weeks of training work up to this level of mileage for the long run.
Walking:
Most use a run/walk strategy and it is better to walk frequently in the beginning of the race, even though it is possible to run more. Every ounce of energy must be conserved so that you can continue to run as long as possible. Alternating running and walking uses different muscles and lets you go further than either one alone will.
Hydration:
Hydration is important especially in hotter climates. The potential for being in the heat for many hours adds to the potential for dangerous levels of dehydration. Drink lots of water and check the color of your urine to make sure that it is not too dark. Drinking only water all day without replenshing electrolytes can lead to a type of "water intoxication", and possibly death. Use electrolyte capsules, such as S-Caps, Endurolytes, or Salt Sticks and follow the directions carefully.
Nutrition:
Assuming an average calorie burn of 100 calories per mile, a conservative estimate, a 100 mile ultramarathon will burn 10,000 calories in a 24 to 30 hour period in addition to the calories you would have burned in a normal day. That is several days worth of food that needs to be consumed in a short period. Practice eating real food with high calorie densities on long training runs and find out what foods work for you.
This is a very popular article with lots of good information in the comments, but I feel like it's getting a little out of hand. The format of the comments makes it very difficult for new readers with questions to sort out the information contained in the comments.