Congratulations on establishing breastfeeding
It's unlikely that your milk will dry up and if you're waking with engorged breasts it sounds like you've got a great supply.
Once bubs a few months old what usually happens is that your supply settles down so instead of having the over suply that you often get in early days you achieve a balance where your body has the right amount of milk for your babies' needs.
When babies have a growth spurt they might feed constantly for a few days - this doesn't mean you have a supply issue or your milk isn't good enough, it's just nature's way of letting your body know that now your baby is bigger she needs more milk.
Congrats on getting back to the gym It shouldn't affect your supply but make sure you replace all the extra fluids you lose when you excercise (same as you would if you weren't breastfeeding).