It's also about pacing yourself. I used to start out going as fast as I could and then when I started to get tired I would put the hammer down just to make sure I was officially still "running". But it actually just made sure I got tired out much quicker, at which point I would dejectedly assumed I'd failed. For some reason it never occured to me to go a bit slower to keep going for longer. But yes, breath control, training your heart and lungs to work harder/more efficiently, and not wasting energy by bouncing up and down too much.
Another important discovery was that fit people actually do get tired doing exercise: getting fit won't stop me getting tired, but it will enable me to keep going even when I am tired. Realising this was a major turning point for me.