Girl I feel you. I was 529 and practically immobile. This is what I recommend. What you need is to just get your heart rate up. being immobile as I was, anything can get the heart rate up. That's great news in a sense because you can sit on your couch and exercise by using your arms, abs, back and core. how? dancing! I love to dance. I actually do zumba 5-6 days a week. However, I was watching my instructor videos on youtube and noticed as I was doing the moves sitting down (lifting my legs even just a little) I started to get warm and my breathing was getting a little hard as well. That gave me an idea. If you find a dance video, zumba video or even I'll give you the link to my instructors videos (she only taped like 2-3 songs) then you can do it a few times while sitting on the couch and get a really good workout.
Also, have you tried shadow boxing? basically sit on the end of your bed or couch, close your eyes and imagine yourself boxing. try this for awhile.
What I did at the beginning was walking. My back hurt SOO bad after 5 minutes it was so debilitating. But I had to start somewhere. I know it seems not a lot but if you can, try walking 5 minutes. that's it. 5 minutes. maybe after 4-5 days, try walking 10. like I said, it might not seem a lot but you have to build those back muscles.
Here is that link I mentioned before. I hope it helps:
http://www.bfabfitness.com/ and go to videos. Doing these moves while sitting will help. Like i said, all you need to do is get your heart rate up. Good luck my friend.