Art DeVany recently said:
If you could only do one thing for your body
You should sprint for your exercise. It is fun, builds coordination, and will make you stronger and leaner. The damage to your joints is far less than jogging and it is more effective than walking. Sprinting trains anaerobic and aerobic pathways, but with less oxydative load than jogging.
Interval sprints, mixed with walking are my favorite way to make my "come backs" when I have been traveling or busy. Even when my joints get a bit sore from working out. Yes, I have reached the age where I sometimes have joint soreness from working out. I find there is no better way to ease the load on the joints and come back to the gym with fresh energy than to sprint and walk.
here is video of Mark Sisson showing what we are talking about