Hi, Jack. Welcome. This is a great place for info and support.
So that's what doctors are saying now, eh? For so long they complained that we got info from the Internet and now they are pushing it on us? Too funny. Or maybe he was being sarcastic, having heard one too many times that we look on the Internet!
I'm with these guys . . . mowing the lawn in FLORIDA?!! You may have to think about getting some neighborhood kids to do it. The heat not only makes MG worse but worse quickly. And when your muscles don't work well, like your breathing muscles, then your heart works harder to get you more oxygen (oxygen stats drop when MG is worse). If you are mowing the lawn while your heart is working harder, you could have a stroke or heart attack!
The thing with MG is that you might be fine now but not in a few months (especially if you keep going out in the damn Florida weather). Or days. Or hours.

MG is stupid. If you push your body, MG will push back.
I am only on Mestinon because I can't take the other drugs until I get much worse. Lots of contraindications. I can manage to be okay as long as I don't do too much and balance activity with rest or sleep. I am on 90 mg. every 3 hours, round the clock. Sometimes increasing the dose amount is right, sometimes it's the frequency. Only your neuro can help you with that. Don't change dosing on your own. My MG crisis, when I couldn't breathe or move well, was after doing too much while in hot weather. Okay, enough lecturing.
I don't know what part of Florida you are in but the BEST MG neuro down there is Dr. Allan Weiss of St. Petersburg Neurology Clinic. He's an MDA Director too.
It's a big learning curve with MG. Drugs can make it worse (see previous posts or websites below). Foods can make it worse or better (nightshade foods like potatoes and caffeine).

I did that just for you, Ally! Those foods do the same thing Mestinon does to varying degrees and it can make you stronger or weaker. They keep away the enzyme the eats up acetylcholine so that your muscles get more of it. Too much acetylcholine and you can get worse.
I hope you can continue to feel ok. Keep in mind that both Pred and immunosuppressants can take a while to kick in. Pred can make MG temporarily worse and has LOTS of side effects.
I'm sorry you have MG but you can manage it. I hope you will not have it get worse but that means behaving yourself.
Annie
www.myasthenia.org (national organization)
www.mdausa.org