I'm pretty sure I'm 65 (bday 3/5/45).

I have the same experience with marathoning. I did 4 of them in 2004-2005, and two of them were in 3:54. But arthritis in my hips won't let me do any more than a mile or two. I feel lucky to have done well for a couple of years around age 60, finishing in the top 4 or 5 in the age group in many races.

I'm glad I can bike without problems (so far). I'm trying to put off hip replacement as long as possible.