3. Lack Of Mobility
As mentioned earlier, stride frequency and stride length are the two components that determine running speed. Check out this video analyzing the stride of Chris Solinsky, the former American record-holder in the 10,000 meters, which shows how hip separation can help propel you forward, and consequently cover more ground.
Mobility trumps all else when it comes to running fast and staying healthy. If you lack complete range of motion anywhere in your lower body, you are going to be more susceptible to injury. A good way to increase running-specific mobility is through Active Isolated Stretching, a method made popular by stretching guru Phil Wharton. His techniques are focused on how to lengthen the muscles properly in order to prevent injury and improve performance.
Lastly, lets take a look at momentum, ideally of the forward variety. One of the best ways to establish forward momentum is to lean from the ankles. This forward lean will also help you avoid running vertical miles. Keep your head as level as possible, and avoid bouncing up and down as you propel yourself forward.