I've been in plenty of street fights and most don't last long enough for cardio to matter. My advice:
Go to your local book store and reed a few books on kick boxing techniques or something. Practice your stance and basic techniques. I'm sure you already have some knowledge because you take Karate but don't try to use that to much in the fight.
What it will really come down to is your ferocity. When you see him don't circle and wait for him to come to you. Rush him. Throw every thing you have at him really REALLY quickly and beat the hell out of him before he even knows what happened.
Also make sure you don't hurt him to bad. When you can tell he's done, STOP. I've gotten tickets for fights I didn't even start. I once had a guy hit me in the back of the head and come at me swinging. I held him on the ground by his throat and only threw a few punches. The result was that I paid a two hundred and fifty dollar ticket and the costs for his stiches.