this English guy wrote:
The 100 km world record is 6 hours and 10 minutes, which is just under 6 minutes per mile. Now, I think the greatest marathoner in history might have the potential to get close to that with proper training. However, this takes time, and I don't think he would be able to get past 50 miles in the next two years. If he (hypothetically) kept training for that challenge, he might get closer.
going from marathons to road/flat ultras, especially a 50 miler, isn't that tough of a transition. Eliud could definitely go run 50 miles at 6min pace after he's recovered from Berlin, with no additional training. he's most likely already doing 24-26mi training runs, which is about all I'd do before a 50 miler aside from maybe a tune-up 50k.
there's no way he'd make it 100 miles at 6min pace though. way too much muscular breakdown plus a hundred other things. running for 8+ hours is more about management, experience, and organization that it is about fitness.
I think Eliud could run 100k at 6min pace, given a few months of notice tops. drop or extend one of his weekly workouts, and extend the long run up to 50k/3-3.5hrs
how much over 100km would be tough to know. it would be a mental battle, very similar to the nike sub-2 setup but at 6:00 pace on a track presumably.