Long distance running is all about efficiency if you want to be good. Having proper form can help you be more efficient and get faster.
So, what is proper form?
Before we go into the specifics I want you to realize there is a difference between form and style. I think every runner has a particular style that is unique and natural for them that should be embraced. Form, on the other hand is about being effective with your natural movement. For the sake of not writing a book here; I will only focus on two particular elements of proper running form that I commonly see problems with and are fairly easy to correct. Those two things are; arm swing and upper body/core strength.
Most runners do not utilize their arms enough. During movements assessments I often find a lot of muscle imbalance with the shoulder joint due to weak rhomboids/back muscles and tight pectorals. This leads to improper arm swing. The arm should swing freely with the hands loose, brushing the hips and swinging even with the shoulder straight out in front of the body. However, most run and cross their hands over the midline of the body. Most runners as they get tired also shorten the swing and hug the arms to the body rotating the torso and losing a lot of momentum with side movement. These things can easily be corrected with some stretching (pecs), strengthening (rhomboids, back and core), and being conscience of how you are swinging your arms. Arm swing becomes particularly important with hill running. So, while you are running be sure and loosen up your shoulder and allow your arms to swing brushing your hand on your hips. When you are tired, “drive” you arms and your legs will go with them. Give it a try and see what happens.
Another thing that can really improve your form and help you run faster and more efficient is improving your core strength. This can be done with resistance or weight training exercises. I suggest starting with simply classic exercises like crunches and push-ups (give our hundred push-ups group challenge a try). Pilates and yoga or swimming can also really strengthen the core. I also recommend following a specific weight training program for endurance athletes at least twice a week. You will become a stronger runner and prevent injury.