It doesn't matter if you're running 2 miles or 20 miles. If you find yourself out of breath the moment your feet hit the ground you're probably running too tense. Try to loosen up your shoulders and chest. Always remember to run tall and relaxed, while controlling your breathing pattern with each stride you take. Never run too tight, too tense, or too stiff. Open up your chest because the more you expand your lungs the more efficient your use of oxygen will be in each breath. Now, go dominate your next run.