what makes you think that?
I think that possibly quite the opposite, your muscle strength and endurance will improve once you stop constantly pushing yourself to the edge.

We are brought to think that exercise builds muscles. And aerobic exercise improves endurance. But, this is not always so and not for everyone. In MG it may in fact be the opposite. At least for some people.

My neurologist's rule of thumb is always do a little less that you can. And never bring yourself to the point that you over-exert yourself.

If you can easily climb one flight of stairs just do that. Don't try to climb two or three or four. This is enough exercise to preserve your current muscle strength.

What have you got to lose if you try for a few months to just keep your activity level relatively constant and not do as much as you can all the time?

I am not suggesting that you lie in bed or sit on the couch all day. Just try to limit your activity level to what is comfortable for you.

I may be wrong, but I believe you will feel better.