A lot of good advice from lots of people. Diet, regular exercise, and staying within your limits are all important. And I always tell people to "listen to your body" as to what you can and cannot do (a little pain is OK but avoid more than that).
What frustrates me most as I get older is that my body used to tell me when I'd had enough when I was doing stuff, but now it doesn't seem to tell me till after I'm done hurting myself! Very frustrating, as I always like to push my limits.
I can still go hard enough to impress a lot of guys who are a good deal younger than me, but now only for a few hours a day, and I hurt afterward. I used to be able to go 12 hours easily with no pain after, so my current condition is frustrating.
I'm still trying to figure out if it is just my age, or if recovering from injuries and surgeries just got me out of shape. Likely it is some combination of both. Guess I'll just keep trying to get back and see what happens.