Hypothetical training musings ahead, by someone who is not a scientist, a coach, or that accomplished a runner. So, get your salt ready:
I'm pacing the last 25 of a 100 this weekend, which is a rewarding experience, but brings with it the unfortunate likelihood that I'll come away from the experience wanting to attempt one of the damn things myself.
I've allowed myself to taste the idea, if only because I don't presently have the desire to make a meal of it.
Still, that hasn't stopped me from thinking on potential recipes I might employ, should a genuine craving strike.
(Food metaphor is done now, I promise.)
The thinking goes that, the longer the race, the more "slow twitch" it is. No need to run fast. The law of specificity says so.
And there's intuitive logic to that. After all, if we assume the most important training is done around and at goal pace, and goal 100 pace is fucking slow (unless you're Zach Bitter, in which case, hi Zach Bitter), then why do anything but indulge in sweet LSD?
I dunno, exactly. Certainly plenty of people (probably most), from the sharpest end of the stick to the blunt, have trained with nothing but lots of weekly volume. Specificity is respected with big weekend mileage, and the pursuit of terrain (and perhaps vertical) that mimics the goal race. That has worked quite well for many.
My thinking, however - and this is far from revolutionary - is that the demands of running 100 miles are possibly best met by including multi pace and resistance training in addition to high mileage.
I believe this might have to do with muscle fibers. For our purposes, let's say that we've got slow, intermediate, and fast twitch fibers. The utilization progression goes mostly how you would expect, as the race distance increases. The further you run, the less fast and intermediate twitch fibers are involved.
And this is true... right up to somewhere around the marathon. At that distance, glycogen depletion and muscle damage require greater recruitment from the other fiber types. And although it hasn't been studied to my knowledge, it only makes sense to assume that 100 milers increase the need for these ancillary fibers to be recruited, when compared to marathons. And so the notion that 100 milers are an entirely slow twitch event, such as it exists, I would argue, is wrong. Much more likely, I think, is the possibility that every damn fiber you can muster becomes vital.
This same concept is also the driving force behind Lydiard's top down approach to training middle distance, which has since been articulated by real runner/scientist Peter Snell.
Do note that, as Snell points out, it's possible to train all fiber types with nothing but lots of steady distance. But you have to do a lot of it, and you can't just jog. Lydiard emphasized most running be done at a "good aerobic pace", which was not slow. And so possible, I'd argue, isn't necessarily optimal in this case. Certainly not when we're discussing the vast majority of ultra/marathon runners, 100% of whom are not Peter Snell.
I know I'm not. So it follows, then, that if I were to train myself for such an endeavor, I'd strive to increase the number of fibers I could recruit, and practice getting them to fire cohesively in a running stride, across a range of paces.
Practically, this means I'd do squats, box jumps, hill sprints, hill reps, track reps, and tempos. In other words, I think I'd probably train an awful lot like I would for sub marathon races, but with a greater emphasis big back to back weekend long runs.
It is worth noting that I'm hardly inventing anything here. Though plenty do train for 100s on nothing but volume, many others do more or less what I just spelled out here. Even still, the "why?" fascinates me, as ever. Maybe not enough to ever test any of this, but we'll see.
(An aside to this: Since it's long enough to not be "fast", but short enough to avoid marathon damage, is it possible that the half marathon is best distance for the truly slow twitch athlete?)