Though it may not seem like it, your technique while running plays a huge role in your exercise efficiency. If you're all over the place, you're wasting a lot of energy that will affect you on longer runs. One of the biggest mistakes I see in runners is improper posture. Hunched shoulders or leaning over forward too much puts extreme stress on your back, not to mention just the work that you're requiring your lower back and spinal muscles to keep yourself erect. Though it may feel akward at first, pull your shoulders back and stay upright. Over time, you'll feel more comfortable. Your back will thank you, and you'll end up being able to get more oxygen into your lungs since they'll be more open.