Like to run? Knees hurt? Back hurt? Hit the sand -and not the hard sand, either. Soft sand only. Do your running in the soft sand and these three things will happen: 1) you'll give your joints (knees, ankles, lower back) a rest from the pounding of hard ground while still working your aerobic system, 2) you'll find that you will have to work up to three times as hard to cover the same ground, in the same amount of time as you would on hard surfaces, and 3) you'll use more muscle fibers running in soft sand-especially core muscle fibers- due to the slight instability you'll experience when the sand "gives" every time you take a step.
It's hard on your aerobic system, so be prepared to slow down to find your pace. I use it to train quick motions in my elite athletes as well. It forces them to work harder to change directions quickly, so that when they do hit the hard surface, more muscle is firing at each point of impact. It's a great workout as well! (brutal, in fact, if you're doing a bootcamp style workout in the soft sand). It will triple those elliptical machine calories in the same amount of time.
Give it a shot, and let me know what you think. Of course, you'll need a beach. And wear your old beate running shoes.