I don't think that riding an exercise bike is better in any way then doing other physical activities (eg-cleaning the closet, or even walking). Did your neurologist specifically say that this is what you have to do?
In fact, I don't think that an exercise bike is good for someone with MG, as it inevitably puts you in a sort of competitive mode. It is also the kind of exercise that increases osteoperosis, as it does not involve weight bearing.
Busy house wives probably do more physical exercise then a trained athlete.
Also, exercise is something that should cause you pleasure, so that you would want to do it. And everyone should find what is good for them.
I believe that what your neurologist meant was that it is good for you to do some moderate physical activity (when you are doing relatively well). -eg, not too much and not too little. I think that cleaning the closet using each and everyone of your muscles can very well account as such. Possibly not doing it in one day, but over a few days would have been better. But, that involves a lot of trial and error, until you find what works best for you.

and yes, the emotional need to do something is a valid need, even if it sometimes carries some price.