I had a friend who was really in to endurance sports for a while. He gradually kept ratcheting up the distance and frequency. At the time, he talked about it as

I had a friend who was really in to endurance sports for a while. He gradually kept ratcheting up the distance and frequency. At the time, he talked about it as chasing a feeling. I saw him once after he'd run something like 35 miles in a day and he was really relieved that he had found his limit at that distance. He was worried that he was going to end up one of the ultramarathoners running 50-100 miles in a day. So yeah, if he's at all representative, I don't think it's health that they're chasing. The most I've done is a couple of sprint tris and a few 50-mile-per-day bike trips. But even that for me wasn't about health; it was about challenge.