So the big question is what to do? How much exercise can you do or should you do to get the results you want? Well that's really a question everyone should answer for themselves. I can tell you what I did in my lifestyle change.
Dr. Atkins says in his book that what's important is to rltry to get in at least 30 minutes of exercise/movement a day. If that seems like a lot and you have a lot to lose then try every other day to start. I decided when I started that I would be true to this and do about 45 minutes of fast paced walking every other day. Fast enough to build a sweat. It was tough when I first started but the more I did it the easier it was to push myself to go further and faster.
After a while I was doing several miles in my 45 minute period. Always walking and always sweating.
Maybe for you it's riding a bike. Or maybe it's jogging. Maybe you do a dance game on you gaming systemat home. What's important is to stick to a time limit and don't be afraid to push. Remember even walking is better than nothing.