I would have to say that form is about the largest factor when it comes to speed and endurance. Often I see bad form that leads to poor energy use, or sometimes even injury. Proper running form should be learned at the very start of your training because if you practice with the bad form, it just keeps reinstating a bad habit that you need to lose.
The Proper Form Guide:
Good posture: It’s important to have good posture while running. It promotes more relaxed breathing. The best way to keep good posture on a run is to imagine there is a string attached to your head that is puling up gently.
Relax your shoulders: Along with good posture, you should keep your shoulders in a comfortable, relaxed position. If your running with stiff shoulders, it will likely cause muscle soreness.
Look straight ahead: While running, it is important to keep your head up and straight. Your head should never wander to the side, up, or down. Any bend in your neck will make it harder to move air in and out of your lungs and sometimes mess up your stride.
Keep your arms at a 90 degree angle: When you maintain a 90 degree angle in your arm bend, it assures that your arm swing is generating from your shoulders. This will create the most momentum. A great way to learn how to always hold your arms at a 90 degree angle is to get a large rubber band and attach one end between your fingers and the other end to your elbow. Now swing your arms through your shoulders and focus on keeping the rubber band on your arm.
Don’t hinge in your hips: When you run, you’re supposed to lean forward, but I often see people hinging in their hips. This is bad because it doesn’t give you any forward momentum and forces your chest down, which makes breathing more difficult. Instead, you should hinge in your ankles. Yes, it will feel like your falling. This is what gives you momentum. I like to think of running as a controlled fall.
Leg motion: Your leg motion is a very important, yet difficult aspect of your form. It’s almost like pedaling a bike. When your foot strikes the ground, it should be directly underneath you. You should then drive your foot into the ground, propelling you forward. When your leg is behind you, you drive it forward through your knee as fast as possible. Your foot should come close to striking your butt. Then your back to the first step, drive your foot to the ground.
Land on the balls of your feet: Where you land on your foot is important. You need to land on the balls of your feet. This will help you generate the most momentum. If you land on your heel, your only slowing down your momentum. It does take quite some time to get use to it, but once you do, you will be much faster.
You will need a friend to help you perfect your form by taking a video and showing it to you. I guarantee your speed will improve if you follow all the guidelines of a perfect running form. Just one last tip. These forms are just guidelines, meaning you should tweak them to make them ft you.