I also have a bad back. What I have been doing in the past week and a half is the Zumba workout. The pace is pretty fast, but I do it at might own pace. When I started, I weight 144lbs with a goal to lose 28lbs. I have lost 5lbs so far. I am not going to lie, my diet is pretty bad. I basically eat anything, but smaller portion. Some gym offer the class. I purchased the discs set ($70)
Ps. I do not eat out. So, I carry a bottle of Ensure in my purse. It keeps me going until I could get home.
I totally understand not being able to exercise due to back problems. It gets so discouraging. try to watch calorie intake and get a low impact yoga dvd. This you can adapt to your abilities. You may want to have your thyroid levels checked also. Hope this is helpful to you:)