I'm not an expert, so some people here may have much better ideas.
I'd say go and buy yourself an exercise ball and some small weights (2-5 lbs), and start by doing small toneing exercises, crunches etc. This you can do in the comfort of your own living room!
This should help you keep your muscle, and if you follow a lower calorie diet you should be loosing weight at the same time!
When you get to a weight where walking is more comfortable, but you're still not up to lots of movement, my mother loves these "Walk away the lbs" DVD's. She says they're a good workout, but still easy to do (she has bad feet). I think she rented them from the library (and then I copied them for her...
), so I'm not sure if they're pricey.
I'm sure lots of other people here will have great advice for you as well!