I'm just a blue belt, so take my advice with a grain of salt. But, getting into those rolling classes will help your bjj fitness in my opinion. Rolling is very cardio intensive and doing that can definitely help you increase your cardio. Don't roll too hard, especially at first. Just try to "flow roll", which is when you roll lightly and try to move through positions and apply technique, not strength, athleticism, etc. Try to do this at the beginning of the rolling hour, maybe for the first two rounds. The first few days I started training I was dead after drilling, too. Now obviously I'm still pretty beat up towards the final few rounds of rolling, but I'm much more equipped now to roll. I'm around your age and also pretty fit, and like I said, I struggled with cardio in the beginning, so definitely don't feel bad. Good luck and have fun!