And while it is not for most people (and definitely not me), there are people I know who thrive on running marathons and the training for it, while otherwise living perfectly normal lives. There is a hypothesis that cursorial / persistence / endurance hunting was a a stage in our ancestors' evolution, and that we have inherited some of the traits of that lifestyle.
But it's mostly a hand-wavy assertion too, and there's other evidence against it. There are cultures around the world (low populations, to be sure) where repeatedly traversing 26.2 miles a day would not be considered particularly abnormal.
Even marathon runners run marathons only on competitions. The chance of injury or sickness after is too high in order to make it worthy. And after, you are supposed to rest for multiple days even if not injured. Again, 26.2 miles is 8 hours of hiking and you are pretty much guaranteed to be ok after it.
Whether any one in particular subscribed to that school of though or not, marathon running is an outlier event, and isn't what most people would consider as part of a regular, healthy, exercise regime that one might do many times a week.