I've never run a marathon. Back when I was young and beautiful I probably maxed out at 20 miles or so training. I've done a couple of half marathon races, though I'm not in any kind of shape to do anything like that at the moment. I think running marathons is a bit nuts, though half marathons didn't seem that unhealthy to me. But nuts compared to what? Once you get in shape (recognizing this stuff gets a bit harder as you age at least if you don't maintain it, and that's assuming your body is going to cooperate at all), it isn't that big of a deal to run one. No I haven't done it, as I said, but once you achieve a level of running fitness. tagging on a few additional miles is usually doable. If you can run eight without wanting to head to the emergency room you can, with some difficulty, run 13.
Anyway, so, yeah, running a marathon isn't one of my goals. I think they are a bit nutty, but people do lots of nutty things. Unless you're an idiot and you're running through damaged joints or similar, they aren't that nutty.