You should try to run at 7 miles per hour for 20 minutes–it gets your heart rate up and provides both cardiovascular and respiratory benefits. I run at the gym on treadmills. The treadmills are equipped with shock absorbers and are generally easier on my joints. You can vary the treadmill speed and slant (elevation) in order to minimize the chance of injury due to repeated motion.
If your joints are not up to running, get on the recumbent bike for a half hour or use the elliptical machine. The recumbent bike is the one where your legs are positioned further out in front of you.
You can also exercise in a swimming pool. The buoyancy takes the weight and wear and tear off your joints. (But I don’t like the chlorine.)
I run 3 times a week and walk for 30 minutes to an hour 3 or 4 times a week, that is, I get some form of aerobic exercise 6 or 7 days a week. And I take calcium (and viatamin D) for my bones and a glucosamine-chondroitin-boron complex for my joints.