As a beginner runner (2.15 half time) how can I use breathing effectively?

Kind of too long for an HN post but long distance running is all a game of efficiency. Good breathing makes sure your oxygen levels are working for you rather than against you. I remember subtle things like switching between calmer breathing to steady the heartrate and deeper faster breathing for getting over a hill. This all can happen naturally, but you'll be surprised the difference in doing the breathing mindfully/proactively rather than passively.

The breathing also helps mentally but also helps relax the body. As you are exerting yourself it can be a natural response to carry tension in your body. Tension is usually the enemy of body mechanics - it leads to inefficient technique.

Watch any sport (including running) and something notable about any pro is how easy or relaxed they look.

This can be learned and breathing has some chicken/egg relationship with relaxation