Don't ignore long distance running. It doesn't have to be a marathon but regular 3 mile jogs are good. In between rounds you get 1 or 2 mins rest. Within that time you need to catch your breath as the saying goes. This will be easier if you have done some long slow runs, basically you'll feel like you are resting when you heart and lungs are operating at the pace they work when doing the long run instead of trying to get to walking around pace.
After a decent base of slow runs work in the program found here.
http://www.t-nation.com/readTopic.do?id=459414 IBUR as the website calls it will kick you into awesome condition for the ups and downs that happen during the round. If you are really doing it all out you will be wrote off at the end.
Then if you can run the bleachers once a week or so to shake things up. After a slow jog to warmup we do bleachers in 3 min rounds. The place we run has 40 cement steps to the top, we run for 3 mins and rest for one, up to 10 rounds.
Even after all that you can get tired in a round, because of getting hit and the full on fury of another person trying to take your block off.
