> One sign that marathons (let alone ultramarathons) may not be particularly healthy is that the first guy to do one famously died, and then subsequently people die doing them every single year

I don't find this line of thinking terribly convincing

It's well known that (even young) elite athletes with unknown inborn heart defects sometimes die after extreme exertion. But it doesn't really follow that "extreme exertion is bad for you," the lurking variable is the heart defect.