Hi melNco -
You can do it!
Don't worry about exercise just yet - they tell you to wait at least a couple weeks before you start. You want to give your body a chance to adapt to the very low calorie level.
I have 2 bad knees, too - ACL repair in 1, chondromalacia in the other. They started feeling much better after just 25 pounds. Now I've just started couch to 5K. I NEVER thought I'd be able to try running!
Good luck to you!