Fighting fit at 41

I have believe it or not taken up boxing at 40 I come from a body building back ground and needed to get fit due to how unfit body builders are I have been training for nearly a year and had 2 3round unlicensed fights which I won I train 3 times a week 1 of which is a personal one on one session with ex pro trainer my problem is although I am physically very strong and can punch hard I seem to lose power quickly and burn out by end of round 2 I'm a useless runner and would like any training ides for me to increase my fitness to be able to blast for 3 rounds solid

And just to add I no longer lift weights and fight at 13st 6

start looking into HIIT.