I don't know how bad your knee is, but I also have some knee issues (1 surgery on each), and I get my cardio in on an elliptical. It's a lot easier on my knees than a treadmill or running outdoors. I still get sore if I go more than 40 minutes on it, but 20-30 don't bother me at all.
Hope this helps.
"He is strong who conquers others, he who conquers himself is mighty" -Lao Tzu