Early 40s, been a runner since junior high. Did the whole cross country and track thing through college, and raced a dozen marathons and countless other road races since.
I am a veteran of one knee surgery and have generally achy knees and hips. Here's what I do to keep running:
1. Watch my weight. I've promised myself I'll never let my weight drift above 140 (I'm 5'7").
2. Diet - I eat a largely plant based diet 5 days a week, and allow for moderate cheat days on the weekends.
3. Cross training - I hit the gym 4 times a week for weights and stationary bike.
4. I take at least one day a week off from running. I go to the gym on those days.
5. Sleep 8 hours a night. I'm amazed at how many people don't get adequate sleep and then complain that their bodies hurt.
6. I minimize marathons to no more than one every other year, if even that often.
7. At the recommendation of my orthopedic surgeon, I take glucosamine condroiton MSM, triple strength Wal Mart brand. He said it can't hurt and it might help, and that the cheap stuff is just as good as the expensive brands. I don't know how much it helps, but I figure it's cheap and can't hurt, so might as well.
That's a lot of maintenance, but I still run around 40 miles a week, give or take, and race 30 or so times a year, mainly 5ks and 10ks. It keeps me on the roads and out of the orthopedist's office.