This is old news. In 1994 in his book The Antioxidant Revolution, Kenneth Cooper, M.D., presented studies that showed that running more than 30 miles per week increased health risks. He was widely criticized at the time, but that was because few people bothered to actually read what he said.
I had the opportunity to hear him speak on this topic back then. To give an analogy, imagine the Olympic ski jump.
Sedentary adults are at the very top of the ski jump with a relatively high risk. The very bottom of the ski jump before begins to curve up represents running 12 to 30 miles per week, the optimal amount of running for optimal health.
Running more than 30 miles per week represents the lip of the ski jump. Yes, it is higher than the dip just before it, but it is much lower than the top of the ski jump.
Cooper's premise was that runners could offset this additional health risk by taking supplements to reduce the damage caused by free radicals, which in turn are created by extreme exercise.