Some of the things I would try if it was me (please note I have no training , am not a trainer, and also have never been unable to walk even at 352 lbs I still walked fairly regularily).
First : Focus on diet, diet not excercise at this point is what is going to get the weight off. And its something that your weight doesn't interfere with, you have lost weight and can continue to lose weight keeping careful watch over your food intake. Honestly that is the MOST important part of weight loss, I find while excercise improves your fitness and ergo quality of life (I excercise even when i'm GAINING weight and I think my life is better for it). Weight LOSS is mostly about food... so that's good news in that regard.
As for increasing your fitness level despite your weight issues...
You can walk a SHORT distance, there are stories about people that just started by walking to the end of their driveway, so pick the distance you can walk without it causing the massive pain.
So if you can walk 1/2 a mile at a time (or however far) without the pain then just walk the 1/2 mile, but say do it a few times a day very spaced apart. So if you can do 1/2 a mile in the morning, without it causing pain, and 1/2 a mile at lunch time 1/2 a mile in the evening. If you keep up a schedule like this your fitness will improve slowly. Eventually when that feels comfortable you can start adding afew meters... depending how you measure distance, maybe adding half a block to your route a week eventually you'll be back to walking a full mile. It's slow and tedious and takes mental toughness but you can do that.
Alternatively or in conjuction try excercise that does not involve bearing your whole body weight. Lets face it at our size it takes a lot of strength and conditioning to keep lugging around all this. Sit or lay on the floor and do moves that work your arms and/or legs. Do AB excercise that involve breath, and simply focusing on thinking and contracting your coor muscles, etc.
So a combination of doing excercises that you can do right now... determination and stick-to-it-iveness, and watching your diet/losing weight you should be able to to more.
For me excercise is about what i can do, not about how many calories it supposedly burns. Excercise is about improving fitness, capability and quality of life for me so I'm pretty passionate about it.
You can totall get to a place where you can do so many amazing things!
Overall Goal (Restart AGAIN) 2013
Mini Goals Celebrated:
310> Feb 2014 - 300> April 2014 290>May 2014