Go to class. Good instructors are open to talking, and if you tell them you're out of shape, they'll help you out as you go along. I remember there were days when I just felt totally beat and couldn't go very hard, and I had other things to do during class instead of rolling really hard.
A word of warning - your forearms will be really really tired from using your grip significantly more than you're normally used to (BJJ) and your shoulders will be really tired and sore from MT. Don't let that deter you, as it's a good feeling!
Suck it up, get to class, and learn!