First off, you probably have a big advantage cardio-wise. You'll have better stamina than the casual students. You'll have no problems doing the warm-up and drilling part of the class.
If you've never done BJJ before, though, a funny thing will happen your first few sparring sessions. You'll come away from your first rolling session thinking that that was the most physically draining activity that you've ever done. You'll probobably feel that your cardio training was not sufficient. (If I'm wrong about this then great! You're a big step ahead of other beginners!)
The reason for this is that most people are totally unfamiliar with fighting on the ground. If you've done wrestling before, it won't be as bad, but psychologically, the vast majority of beginners get really worked up. They are super tense when they roll, and as soon as they get put in a bad position (e.g. someone gets them in a very tight cross-side or knee-on-the-belly) they tend to waste a lot of energy pushing in all directions in an effort to escape. Your heart and breathing rate shoots up, but it's very difficult to breathe because your training partner has all his weight on your chest.
The good news is that once you get a bit of experience (and have learned the essential positional escapes), you learn to relax. You learn where to focus your pushing so that you don't waste energy. Once you get to that point, your boxing conditioning will give you an advantage.
For other kinds of conditioning, Hardball is right - work on your flexibility!
Check out this website - there are a lot of great training tips here
http://www.grapplearts.com/index.htm
Especially
http://grapplearts.com/Starting-BJJ-Classes.htm
Good luck and enjoy your training!
Edit: Yeah! What Danfaggella said!