Yes yes yes, stick with the yoga. Remember that when you see an instructor do a pose, they are modeling the "ideal" version. The beautiful thing about yoga, though, is that the point is not to do the ideal version right away, but do what you can and go from there. It's definitely true that, due to my excess flesh AND my bad knees AND my bad balance that there are some poses that are not available to me at all at this point. And, there are a lot of poses I can only approximate. But, it doesn't matter whether I can "kiss my knees" or not in a forward bend -- what matters is that I maintain proper alignment and move as far forward as I can. I get the same benefit as someone who can fold all the way forward. The benefit doesn't come from being able to do exactly what the instructor does -- it comes from aligning your body properly and doing what you can. The whole philosophy of yoga is honor your body, and do what you can. Even with all my limitations I would not miss my yoga class for anything, and it has helped me not only physically but mentally as well. It has informed the way I move through the day, it has helped me with other types of exercise.
I really highly strongly recommend the book Cool Yoga Tricks (
www.coolyogatricks.com) by Miriam Austin. It does a great job of explaining a good set of standard yoga poses and all the ways you can use props to make them more available to you. She also wrote a book called Yoga for Wimps that is sort of a pre-yoga instruction manual. You may find it helpful to start there, then move back into the video. Another good video is by Gaiam -- Yoga for Weight Loss. It also includes lots of modifications. It's especially helpful in DVD format if you have a DVD player.