run in intervals. If you're on a treadmill what I like to do is change the incline or speed up half a notch every single minute. Just so the "feel" never gets into a rhythm. There is NO rhythm when you're fighting someone, boxing is completely random in there and its incredibly hard on your body. Sparing 5 rounds is probably just as bad as running for 1 hour. If you're on a treadmill try changing the speed and incline back and forth every minute or two and that will really work you. I also like to just start off at a high speed and when I need to drop it a notch I'll slowly phase out the speed every 2 minutes. But I mean slowly so even though you're decreasing the speed since it is such a small decrease it will not feel like much of a change because you are progressively getting tired. Running at your max builds your mental strength too, nothing is worse than being at your heart rate max its an unbearable feeling but thats how you build your endurance.
i run endurance cuz i do a lot of halfmarathons but i like all aspects of training so i do hiit aswel