Trying for a formal exercise program may just be too much. If you are exhausting your muscles to where you are weak and in pain, that's too much. I just do what I can, when I can. Take the dog for a leisurely walk (he likes to sniff every blade of grass on the way anyway). Sit in a comfy chair and do the arm movements of the Tai Chi I used to know (a little) for just a few minutes, so I don't get to the over exhausted stage. Lots of little bits work much better for me than any extended or intense activity.

If your MG is affecting your leg and body muscles, then exercising isn't going to fix that. It would be like thinking that you can make your car stop running out of gas by driving it more. Nope, when you're out of gas, you're out of gas, and you have to stop driving until you get a refill. When your muscles say "I've had enough now", working them longer won't help, giving them a rest to "refill" will.

Definitely talk with your doctor about what's going on. If your doc comes back with one of those "no pain, no gain" lines, find another doctor. You can't fix MG by just trying harder. You'll need a doctor who understands it and knows how to treat it and work within it.