I think running 5mph for 5 hours is a very ... achievable goal for the median person, but I don't think I would put money on the median person, or at least, American, completing that time without additional training from their current condition.

If you took a random sample of people -- or at least a random sample of people 18-50 with no outstanding health issues -- and put them on a "couch to marathon" 6 month training program, yeah, I think you could plausibly get a median finish of 5 hours (for comparison, the Boston marathon has a median finish time of a little under 4 hours).

I honestly wouldn't bet on the median person with no training getting off the couch and moving 26.2 miles in less than ~10 hours.

I don't really have much of an idea of what the median would be for time required to walk/run 26 miles. I doubt that there's any good data. The original comment just specified "men", which is presumably global and would include the elderly. I've read that about 45% of people are non-urban, which may be expected to increase fitness, perhaps.