10 minutes or two laps of nice easy running.
Then do at least 5 minutes of dynamic stretching and mobility.
2 laps (800m) breathing only through your nose.
1 lap (400m) just running easy and breathing normally
1 lap (400m) pushing your pace. Notice what happens to your breathing.
Return right away to the top of the set and repeat…
2 laps (800m) settling down and return to breathing only through your nose.
1 lap (400m) just running.
..and so on.
If you have time and have already been practicing nasal breathing, repeat this progression again.
You will have to run slowly. It will feel very weird! The hardest part is trying to calm down and return to breathing only through your nose after pushing your pace. This is really great practice for pacing and for settling down in a race when you get a bit out of control.
Your body is building up CO2 when you limit your air intake like this and that forces your respiratory tissue to relax and the muscles to get stronger. With regular practice you will be able to have more control over your breathing and calm yourself intentionally and effectively.
Eventually you may find that you can run faster and longer while maintaining this calm, controlled breathing.