Ooh, I have the AM/PM yoga tapes, too! I love them! When I can't do a position (bad knees), I modify it a bit and just focus on my breathing and balance. The body-awareness yoga brings is amazing, and as clumsy as I am, I definitely need it.
Fit TV used to run a wonderful yoga show; I used it every day for about a year, then sadly stopped. If you get that channel, check the listings; it might still be on.