Most experienced runners can tell when other runners are in shape just by watching them run. They look for efficiency, a measure of how much energy is lost by wasteful movements during running. You run with your legs and all of your other movements are used just to balance your body. The main reason you don’t fall when you are walking or running is that your brain constructs a "center of gravity", a point around which all movements on one side are balanced by equal movements on the other side. For example, when your right leg goes forward, your left arm goes forward and your right arm goes backward. You do this without thinking and your movements are automatically calculated in your brain.
A study from The Hospital of Laval in France shows that even the best runners lose their efficiency when they become fatigued ( Journal of Sports Medicine and Physical Fitness , June 2006). Experienced runners have consistent stride length and form. This study shows that with fatigue, their stride length decreases and, more importantly, they start to lose form by adding a significant amount of side-to-side movement that wastes energy and does not drive them forward.
Running slowly does not teach your brain how to balance your body when you run fast. Good form comes from practicing running very fast in training. People who run slowly all the time usually have poor running form. They waste movements that do not help them move forward. For example, their feet often move to the side after they raise their feet from the ground. Their arms do not move loosely and comfortably to balance their bodies. They may run with toes pointed outward, which is a sign of weak shin muscles. If you want to improve your running form, run faster a few times every week. More