I forgot to mention in my first reply that I, too, have a bad knee. Had to quit running a LONG time ago, and the knee still bothers me on hikes and stuff. But, I can bike! I ditto all the advice about using low gears to spin easy and really build up slowly. Now, I can handle muscle tension workouts with hard gears, but a couple years ago, they hurt and I couldn't do them.

And, I also ditto what someone said about just grinning, still, on a good ride. Yesterday, I had a great ride, and people driving by must have thought I was nuts, because I just kept breaking out laughing I felt so good, and so happy, to be out there.