I was doing physical therapy for knee pain when I starting riding my bike. That was about 18 months ago. The doctor and the physical therapist said that biking was a great idea. My knees are arthritic, but building up the muscles around them helps a lot. I also wobbled at first, and had to get off and walk the bike to get up the little hill my house is on. Now I can get up the same hill without being in low gear! I feel a lot better, and my knees thank me every day. Last month we went on a bike trip in Europe, and I rode 30 miles every day for a week, and I wasn't exhausted or the slowest in the group! Yay!
I think you've already gotten some great advice from the ladies here. I just want to encourage you to keep getting on the bike and doing what you can, whatever that is. Some days it's hard to get started, and I'd rather make up excuses, but once I get out there, I'm always glad I did.

Kris