I'll play.

Years cycling: Off and on since 1985, regularly since 2002
Age: 55
What keeps me on the bike is that it's one of few exercises that I truly enjoy (other than yoga, but that's a different kind of thing). As someone else said, I can either challenge myself or just twiddle along and smell the flowers if I want to, or any combination of the two. Whereas running, well, I did it for several years but I never fell in love with it like I have with cycling. Also, I've done a couple of multi-day bike tours, and those were wonderful. The Chief is not a cyclist, which is too bad; on the other hand, we're not looking to be conjoined twins, and cycling provides some quality "me" time.

My weight has been all over the place for years, and it's still not under control. BUT, for an overweight middle-aged woman, I'm in pretty good health--my resting heart rate rocks! In fact, I started a weight-training program just this morning, and the trainer asked me if I'd made an error when I recorded my resting heart rate! That made me feel pretty good--and I have cycling to thank for it.