There are a lot of seated exercises you can do with weights, and probably even some cardio.
Do you know exactly what is wrong with your ankle? Or is it just giving you pain? If you're not sure of the exact problem and can't afford to see a doctor at the moment, I'd suggest some home physical therapy activities for it! It could really help. I had a bum knee and ankle, and noticed that once I started getting in shape and lost part of the weight, the pain went away and I was able to move on from walking to running and even more intense activities like dancing that included lunges and all sorts of things.
Look at some of the exercises on this website... they are basic to help strengthen legs, calves, feet, ankles... most people who have pain in these areas that isn't related to a previous sports injury like a break or a sprain... it's just a matter of lack of strength and endurance:
http://www.active.com/running/Articl...or_runners.htm
I would recommend doing these exercises along with slowly building up to longer walks... start with 10 minutes for one week... move to 15 minutes the next week. Do the stretches after the walk, and then elevate your feet for a little bit to keep them from swelling. There are probably a lot of simple things you can do without needing to spend money on a doctor. With some weight loss also you'll see less stress on the legs and all the joints.
PS! Join us over on the 20-somethings board!