2 Blowhards pointed me to a list of 10 reasons. Here is a sampling.
Marathon running damages the liver and gall bladder and alters biochemical markers adversely. HDL is lowered, LDL is increased, Red blood cell counts and white blood cell counts fall.
Marathon running causes acute and severe muscle damage.
Marathon running elevates markers of cancer.
I personally need no convincing, not having done any distance running since the Carter Administration. If this is to be believed, Dale at Faith in Honest Doubt should be dead by now.