I was a decent recreational runner in my late 20s. But pain tolerance is not "the difference", just "a difference". I do not have the classic marathoner's frame, and at above 60 miles per week, I would start to feel twinges here and there, which likely would've developed into injuries had a continued to train at that level for more than a couple of weeks. Now, 60 miles per week is nothing for a marathoner, about half what the true stars run.
As with any form of highly competitive athletics, the best advice is to choose your parents carefully.
As with any form of highly competitive athletics, the best advice is to choose your parents carefully.