I started running while training for the cross-country ski team in high school and college. After a few years of dreading the first few weeks of training in the fall, I decided it was better to train all year round. It was initially a strategy to avoid the pain of those first few weeks in September. It then became a habit, and even a source of pleasure. It's not just the runner's "high"; it's the opportunity to feel my muscles and bones and tendons all working together with my heart and lungs. Call me crazy, but it's kind of amazing how all that works and I really enjoy the feeling of being more in touch with my body then when I sit behind a desk or walk up and down the halls of the hospital. Add to that the pleasure of the sights, sounds and smells of our gorgeous Vermont countryside, and I find it something I look forward to. 
So, like all baby boomers, I'm having trouble adjusting to the fact that I'm no longer 25 years old. I've now had each knee operated on to remove a shredded portion of the cartilage. Knee arthroscopy (the ‘scope’) is quite amazing in a skilled orthopedist’s hands. Each time I was walking the next day and running in two or 3 weeks. However, Dr. Ketterer, my orthopedist, patiently reminds me that I need to cross train and find other sources of activity that are a little less traumatic to my remaining cartilage. When I took up tennis, he commented "that wasn't exactly what I had in mind". He suggested a bicycle, or if I must run -- do it on trails in the woods where the surface is a bit more forgiving and I might get a few more miles out of my knees.
Well, impatient and time pressured clod that I am — I ignored him. However, my wife managed to find a solution that really changed my outlook. I hate it when she does that, but being from Mars does mean I can occasionally learn from Venus (Men are from Mars, Women are from Venus).
So she got a dog, over my loud protestations about dog hair, chewed shoes, and the like. And the dog simply loves the woods! So now instead of figuring out how to fit in walking the dog and running, I combine the two. Or even add a third goal -- spend time with my wife, when she joins us. So we go trail running; the dog makes me stop now and then for a jump in the stream, chasing deer scent, or rooting out chipmunks. I am more relaxed, the dog is ecstatic, my knees are less painful.
So boomers -- get a dog!