lol well I'm just back from gym, did LesMills Grit Plyo (brutal!) and did bodypump yesterday, and still alive, so I personally didn't see a reason to wait til the check. You should be ok too, if you feel fine and especially if you are not massively fat and desperate to slim down slightly nuts like me and start with some more gentle classes.
However, you do need to be careful with ab exercises if your stomach muscles have separated, and should do some specific exercises to target that problem. To determine if you have diastasis, lie on your back with both feet on the floor and your knees bent. Place the fingers of one hand just below your belly button, lift your tailbone toward the ceiling, at the same time lift your shoulder and neck of the floor. Holding this position, gently press down to feel the gap between the abdominal wall. If that gap is more than two fingers wide, it is diastasis.
As for expressing, I usually do it at least once a day, the amount I get varies depending on when he last fed, usually can get 2-4 oz. We've only tried the bottle once and he ate 4 oz then. At the moment the plan is to use the bottle only when I'm not here, I'm building up the freezer stack with the rest of milk.
I have Ameda lactaline double electric pump and Medela Calma teats.