Breathing again. This is why the Army sings while they run, to ensure everyone exhales fully.

Try counting breaths, 4 steps out, 4 steps in. Do it before you feel discomfort, either breathless or pain.

If 4 is too much or too little, use a different count, Exhale 2 3 4 5, Inhale 2 3 4 5. See how far you can work up the count. (Some people get to a dozen pretty easily, some can do 20 counts with practice.)