Which is the myth? That Pheidippides died (or even ran the first marathon) or that marathon running isn't particularly healthy?

While I believe that good cardio is important to overall health, I think that running a marathon (and training to run a marathon) is not a particularly healthy endeavor. When you start putting in really serious training, like 15+ hours of cardio a week, hormone levels will often go pretty whacky. That alone has to be telling us something.