I don't think anyone with street fighting or ring fighting experience thinks that a street fight is going to go on length wise like a prize fight does, but your body holds up to the amount of training you have done.
If a runner is training for a 5 mile marathon, he runs 10 miles.
If a swimmer competes by doing 15 laps, they practice doing 30 laps.
Better conditioning most always determines the outcome, no matter if it's a 25 minute combat sport match or a 30 second street fight.
Most instructors who tell people this are fat f**ks who can't walk a set of stairs with out getting winded!