"Because I'm crazy." That was the answer I recently gave a doctor who asked me why I had done a "full distance" triathlon. He was just curious, I think. Or maybe he wanted to make sure I didn't think it was a particularly healthy activity. My answer wasn't really serious. I do these things because I enjoy them, despite the difficulties involved. But that definitely puts me at the far tail of the exercise-distance bell curve.
I'm sure they exist, but I haven't met a doctor who would recommend 31 mile runs or 112 mile bike rides. Doctors tend to recommend much less extreme forms of exercise. They've got lots of studies proving that various levels of exercise are beneficial for various conditions. They figure that anything with the word "extreme" in it should be approached with caution.
Individual results do vary. There are people who live to 100, smoking and drinking and doing hardly any exercise. And there are teenage athletes struck down by heart problems during competition.
I have been involved in at least 4 races where someone died. 2 triathlons, 2 marathons. It's very sobering. They were very big events, which increases the odds, but still!
Earlier today, in a comment, I mentioned that I'm not an advocate of running for others. I'm not an opponent, either. I don't even tell people they should "get into shape" or "exercise more" or anything like that. I figure these are very individual decisions, with lots of factors involved. People have different bodies, different situations, different preferences.
There's a humorous saying: "Exercise. Eat right. Die anyway." This I believe.
I just find satisfaction
In certain forms of action.