I've had bad knees since childhood; one replaced 6 years ago & never got full mobility back (my doctor said 'Hmmm; maybe I shouldn't have cut that tendon..."
). Trying to keep the other knee healthy. I do water aerobics - it's a great workout & easy on the joints. I do strength training for upper body & core (can't do lunges or kneeling stuff) & that works great too. If I can't get to the pool I do dancing for cardio but it's pretty low-inpact; still gets my heart rate up.