Vivienne,
Don't despair. If you go to all the rehab they offer you after the knee op/s you will regain mobility quickly. A friend of mine was unable to do much before her bilateral hip and knee ops. When they offered her intense rehab she used it as a way to begin a new exercise routine that she continues to this day (3 years on).
I guess what I mean is don't get disheartened, think of it as a good way to make a new resolution and to begin a new, more active lifestyle.
I would check out swimming first. Whilst you don't put any weight on the joints swimming is very strenuous for the knees whatever stroke you use. Just check with your GP or consultants office that you will not be doing more damage by swimming.
You could consider doing some body conditioning/muscle work instead of aerobic work until after your op. That way you could tone and sweat a bit without having to use your knees at all. It will still help with your overall fitness and will aid your weight loss too.
If you get back to me I will send you some exercises that are easy to do at home.
