When it comes to breathing while running, you need to focus on the rhythm. Coach Jenny over at Runner's World says you can even think of it as a dance between the two elements if you'd like.
You should match your breathing to your steps. For an easy-paced run, inhale for three or four steps then exhale for the same amount. Count the steps in your head while you adjust to breathing on tempo. If you are running more intensely, your breathing tempo will increase to support your increased energy output. So the rhythm is more like a breath in for one to two steps and out for one to two steps. If you can't match your steps to breathing tempo then you are trying to run too fast, slow down, and get back into your rhythm.