A lot of people have good things to say about running for cardio.
I however never run and when I was fighting had excellent cardio, I achieved that by using a combination of hiit technique and steady state application of my discipline which is kickboxing.
Being able to kick 50+ times in a row then throw a hundred hooks, then a hundred straights as a round and to then repeat 5 times while training for 3 round bouts, for example, did me fine.
We nailed a good 30 min of squats, push-ups, burpees etc as a warm up.
The rest is being relaxed and breathing while fighting. That will really pay you the greatest dividends.
The final piece is to arrange your pace to match your style and to give yourself ways to catch your breath which are occluded to the opponent. I often used my jab being always the taller fighter, I trained my jab under exhaustion more than anything else and used movement and feints to back it up. They of course only worked because my opponent was already afraid of my power due to me setting the tone early on.