Use 2 to 1 breathing when you exercise and you will not get tired. If you can run 5 miles, you will be able to run 10 miles using this breathing. Make sure your exhalation is twice as long as your inhalation. You can pace yourself with your steps as someone previously mentioned.
By exhaling twice as long you expel the toxins that accumulate during exercise at a greater rate than regular breathing and as a result you can last longer.
The science of breath is the highest science.
- abir
You must breath both through the nose and mouth otherwise you will not have enough oxygen.
Breathing technique during exercise should come from the diagphram and not from the chest. You should feel your stomach expanding and contracting. If you breathe with the chest and shoulder you will not get enough air and you will be tight in the shoulders.
Also you should try to breathe in a rythmn. For example I take a full breath (inhale and exhale) about every 4 to 6 strides depending upon how fast or hard I'm going. A relax run is 6 strides, and a pushing it run is 4 strides.