The problem: One of the most common injury-causing running form mistakes is overstriding, or landing heel first with your foot well ahead of your body's center of gravity. Some runners assume that a longer stride will improve their speed or running efficiency, but that's not the case. Overstriding wastes energy since it means you're braking with each foot strike.
The solution: Make sure that you don't lunge forward with your feet. Focus on landing mid-sole or on the balls of your feet, with your foot directly underneath your body with every step. A short, low arm swing is the key to keeping your stride short and close to the ground.
More: What Part of My Foot Should I Land On When Running?