I like the treadmill too, but I run on it instead of walking. Walking on a treadmill is just so damn boring. Atleast with running, it is a challenge and you can always try to go further and faster. Plus you are burning a lot more calories faster.
I find that long distance running burns the size off my legs, so be careful with that if you are trying to keep your quads intact. Ever seen a runner with huge quad sweeps?
Walking, interval training, and sprinting are better options IMO than running. You can make any of these as interesting, exciting, and challenging as running too.