I used to use a treadmill, lately I've switched to the elliptical machine for my cardio. It is a smoother motion, doesn't jar the knees and ankles nearly as much. Plus, I can turn up the resistance for a better workout. I use one of those that works out your upper body too, so it's a good aerobic exercise.
I do not own one, I use the equipment at the gym where I have a membership, so if you are looking to buy I can't help much there.
Age and treachery trumps youth and exuberance.