Given someone was training for the 5k and doing around 75mpw with a 15 mile long run, how difficult would a 50 miler be for that person? Would the lack of longer running seriously impact their ability to run an ultramarathon? Would they totally gas out at 30 miles even with with proper nutrition?
Nutrition and cramping would be the problem. 75 mpw would be fine although 15 miles would be way lower than you'd want. 50k would be okay with only 15mi long runs but 50 miles really requires more long runs.
You could finish, but you'd want to get some nutrition advice in advance and get used to taking gels.
The hardest thing would be to run the proper pace. If they can do that, then the next obstacles would be eating and nutrition during the run and muscles cramping because of sheer exhaustion. A couple weeks before the 50 mile, I would run an easy 25-30 mile run to practice nutrition and get the muscles a little more used to going for an extended period of time.
Most runners doing 50 milers are probably NOT training 75 miles a week anyway.
Depends on how they pace themselves, course and nutrition. If said person is a sub 15 minute 5k runner, and they go out at a 8 minute pace over a flat course it wouldn't be that hard.
That's what I was thinking 8 minute pace seems more than manageable for a person of that fitness, given nutrition and pace are proper.
I thought most Ultras were glorified hikes? You'll be walking the climbs no?
I think the answer depends on the runners history. Have they been running 70+ miles a week for years and completed a lot of lifetime long runs? Are they young with little experience with long runs? The 50 miler would be difficult for anyone who gets after it on the run. I suspect the runner training for the 5k would go too fast the first half and suffer the second half of the 50. The" too fast" pace feels sooo easy until it doesn't.
I agree with want everyone has said but I'd like to say that the terrain could be something that messes up the 5k runner. If it's on trail with hills it could give the fast 5k runner problems with the legs and maybe ankles. All he'd have to do to get prepared is some long (3 hours or more) slow runs on similar terrain. The 75 miles per week shows he's capable but you'd still want to do at least one long run on as close to race terrain as possible.