Well I'm a firm believer in and a good example of "if you don't use it you loose it." (I use to teach H2Ox classes need to get back to that) So I don't recommend giving up exercise, but maybe find another way to exercise. If your in shape, you can now maintain that without all the stuff at the gym. (You might have to go to the gym just a few times per month as opposed to 2-3x's per week) Lots of people do but you'll have to be creative. There may be other factors involved in her not wanting to go to the gym...for eg: most people not in shape find the mirrors at the gyms to be humiliating. Sooo...Find a way to workout together if thats what your wife wants, biking, hiking, walking, whatever you think she might like. You probably will have to slow your momentum down in such a way that its not too noticable to her, so she doesn't feel like she's olding you back. Try jogging a mile first, LOL, before you go together. Once she starts exercising with you she might feel a little better about herself just from the effect of the endorphins alone.
Don't know if this would help but maybe there's an idea in there you could employ....