Hey, Angell. This is a tough topic for MGers. How much you can exercise depends upon how "stable" your MG is.
All I can tell you is what I attempt to do. I have a total gym, a stationary, recumbant bike, and a version of the first product below.
I have found that more "fluid" resistant exercise is better for MG, instead of using weights. A pool would be idea—literally fluid—if you can find one to use. A pool cools down the body at the same time you are working against the resistance of the water.
I alternate the aerobic nature of the bike one day, with total gym type reps two days later. I don't do something every day. And it all depends upon whether or not I've gone out to do something. Going out really wipes me out, so nothing is done for a day or two.
MG is weird. You can do something one day, feel slightly worse that day, worse the next, and even worse the second day!

But everyone is different.
There's a lot you can do with diet, too.
I would also suggest getting an oximeter so that you know what your normal O2 is, normal pulse, and what happens to them when you exercise and afterwards. Also, have a fan blowing while you do exercise, so that your body can remain on the cool side. That is really important.
You might need to rethink exercise, but that doesn't mean that you can't have some kind of program. I would also suggest speaking to your neurologist about what she or he thinks about it all.
I hope you can continue to exercise or, rather, mini-exercise.

Annie
http://www.hsn.com/products/tony-lit...t-dvds/7220265
http://www.qvc.com/CatalogSearch?lan...word=total+gym