I was hit by car in '94 and was diagnosed with chondromalacia patella too. I've logged thousands of miles on my road bike - less so than my fixed gear which I've only had for 3 or so years and I have been fine. I run a 40x16 to get me up the San Francisco hills - which even in that low gear sometimes still requires a stand and grind. I do some occassional running when I don't have time to ride and that seems to aggravate the knee the worst. I read somewhere that cyclists suffer from softer bones as opposed to runners. Something like there isn't enough pounding on the bones in cycling which doesn't toughen them up. But I digress. Net-net, whether it's cycling or running - it's better than sitting on my couch w/ the remote and chips.