Many experts believe you should breathe in through both your nose and mouth. Your muscles need lots of oxygen, so it makes sense to maximize your intake. Also, when you’re running (especially at a faster pace or in a competition), I believe you should breathe out harder that you breathe in. That feeling of “not getting enough oxygen” may be caused by not getting rid of gases already in your lungs. During normal workouts, the general rule is that if you cannot talk, you’re running too fast for your fitness level. This is known as the “talk test”. Your breathing should also follow a natural rhythm that coincides with the swinging of your arms. And like most other aspects of long-distance running, it should be even and consistent.