I used to run and had to give it up because of knee problems too - patella something-or-other. My other hobby is hiking, so I found a treadmill that has a 25% incline. Instead of the pounding of running, I can go at a slower pace (2.5 mph) but at a 25% incline, my heart rate is as high, or higher than with running. I've found it to be a good alternative to biking, and it seems to work out different muscles, so I think it's good for cross-training.