There's surprisingly little written about how to train to run 100 miles. What there is is vague, misleading and contradictory. If you ask 100 people how they train to run 100 miles, you'll get 100 different answers; if you ask them how you should train, they'll tell you that you have to find what works for you. After a couple of years of looking at the distance, I've boiled it down to two training principles.
1) To finish 100 miles, train like you would for a marathon, but with longer long runs.
This principle is so simple and obvious, it's easily overlooked. Whatever gets you to the finish line of a marathon will help get you to the finish of 100. Faster runners, just like in the marathon, do more miles and more speedwork. After examining every crazy idea anyone's ever had about what one should do, it turns out 100 miles is just like every other race, just longer than most.
The question then becomes: just how long should those long runs be?
2) Do whatever you have to do to get to the race feeling confident and determined to finish.
It's a good idea to run a 50 mile race before running 100; it's a good idea to run a few of them, but it's not absolutely necessary. It's common to wonder "How do I know I can run 100 miles, if I've never run more than 50? Should I maybe run 75 in training?" Do what you feel you have to do.
A great example of this is Julie Berg's experience at the Superior 100. I think her first time there is the only race she didn't finish; she kept falling over tree roots and quit at 77 miles. Afterward, she talked with some "experts" - anyone who'd finished it - and she was convinced that the endless hills had been too much for her legs. Paul Hasse told her that if she ran up 300 foot Buck Hill 30 times, she could finish Superior. She went out there every week, progressing from 5-10 hills at first to doing 30 (maybe even 35) more than once. Everyone who saw her that summer said she seemed like a woman possessed. She got to the start line of the Superior 100 that fall confident and determined. She finished easily.
This is where I got "Steve's Rule" of running 1/3 of the race distance with 1/2 of the climb in 1/4 of the time. 34 miles with 10000 feet of climb in 9.5 hours and you can theoretically finish Superior. For my favorite training hill, that'd be 43 times up - I once did 30 times in 6 hours (43 would've been just barely possible in another 3.5 hours) - that would be a very hard workout.
I failed my first three times at 100 miles. The first time I had diarrhea (76 miles). The second I had swelling and blisters (first blister problems in 15 years) (85 miles). The third I shouldn't have tried; I was a physical and emotional wreck (58 miles). The fourth time, I had been training to run a 50K in under 4 hours and decided three weeks before that race to run 100 the week before it [good decisions have never been my strong suit]. I knew people who had finished 100 miles who not only weren't athletes, they weren't even healthy, so I kept thinking, "If they can do it, certainly I can." I had run 50 hilly miles in training at a temp of 15 degrees and knew I could do that on any given day. If necessary, I could walk the last 50 miles and still finish.
That's exactly what I ended up doing. I walked the last 50. I went from first place to last. All the while, people asked me how I was doing and I always answered, "I'm going to finish." Confidence and determination, nothing else. I finished the 100 and then ran the 50K the next week in 5:00. It's not the plan I'd recommend to anyone else, but it helped me understand everyone who said "you have to find what works for you."
5 days ago