Make sure you work on cardio AND grapple. On the days you don't grapple, run. On the days you don't run or grapple, drill. I ran 2-3 miles on the elliptical for a month twice a week before starting BJJ, and although that prep was a coincidence (I didn't know I was going to start grappling when I did), it wasn't enough. It will take a few months to get up to speed if you are out of shape; however, nobody will think worse of you for it. Well.... only the jerks (but there aren't that many IME).