I used to be a runner before knee problems brought me to cycling. Lately, I find that I can run/walk without any knee pain, as long as I continue to mix in cycling and stretching. I read a book by Jeff Galloway called Run/Walk/Run that caused me to give it a try. I think that the alternating between running and walking prevents overuse injuries, since both movements use slightly different muscles. Mixing in cycling makes it even more effective. Plus, the running part of RWR really helps get the heart rate up.