William, I agree that it's only habitual marathoners that destroy themselves.
My boyfriend's running his first marathon in December and not only is he incredibly fit with absolutely no running-related injuries and very little fat and a nice dose of muscle, he's also pretty damn healthy. He follows a running schedule but adapts it if his body tells him otherwise. He lifts weights too. He has a pretty decent diet (not perfect, but decent), gets a lot of sleep, has a great attitude (very positive mental outlook), works a relatively low-stress job, spends plenty of time outdoors, has a good support system, etc. He eats a lot of fruits and veggies so is hopefully getting enough antioxidants through that and after reading an article I sent him he wants to get on CLO.