It may be the damage of repetitive motion, it may be chemicals released into the bloodstream from endurance athletics. It may be something else. Without knowing the root cause, it's impossible to figure out the "sweet spot"
The marathon runners I know also seem to eat tons of junk food, they can get away with it from a weight perspective because a long run will burn it off, but it could have other consequences.
Point being: there's a lot about long distance runners that's quite different from other people.