My exercise level went from hiking 2-3 4ks a week to none over about a 4 month period, as MG ramped up. Now that i'm on proper meds i do about 8-10 miles a day on the stationary bike but only on level 2 (practically no resistance), used to do 15 miles on level 7. Likewise i can go on 1-2 mile walks as long as it is mostly flat, anything uphill i'm winded in short order. It is nothing in comparison, but I can do light exercise, i just have to watch my breathing and take breaks as needed.
But .... MG effects people differently so some people can do more while others are more limited. I have to say for me, switching my infusions to vyvgart helped, originally i was on IVIG infusions, but that was for me, also different drugs work better for some than others.