It's rare but can happen where long distance running causes ischemic colitis which is where on a long run enough blood is diverted from the large intestine that it damages the intestine long term. It isn't surprising to me that there's higher likelihood of colon cancer given this. It seems like repeated bouts of lower blood for the intestine on long runs has a cumulative impact and damages the colon even if it doesn't cause ischemic colitis.

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"

It's not simply endurance athletes though. It was 2x ultra-marathons >26 miles, or at least 5 marathons completed.