We have to breathe to live.
But this subconscious act is more important than you think for running performance.
Your breathing is a limiting factor. Too often the exhaustion that slows runners comes from the gasping for oxygen rather than the build-up of lactate in the legs.
The rhythm of your breathing reflects your level of effort and is an accurate predictor of future running pace. If your breathing is not under your direct control then you are running too hard to sustain and the pace is likely to drop quickly.
Whenever I concentrate on my breathing whilst running I take air in through my nose for two seconds, then blow out harder through my mouth for another two seconds. This means I am better in-tune with my body as focusing on my breathing inadvertently controls my heart rate, and therefore my running economy.
Your mission should be to run faster whilst maintaining a regular breathing pattern. Therefore, in training you need to experiment to understand how it affects your specific pace and enjoyment. Over time your desired rhythm will become instinctual.
The better you understand your breathing, the more you can channel your nerves and excitement into improved performance. Regardless of the running distance it is essential you start in control of your breathing, so you still have enough puff to finish strongly.