There are two ways to generate speed when running. One is to increase stride length and the other is to increase leg turnover.
Leg turnover simply means the number of times your feet cycle through the running motion (or touch the ground) in one minute. Everyone has different stride lengths and although this can be improved it is typically very individual and can depend on a number of factors such as leg length, flexibility and range of motion- as well as range of functional motion (the maximum range you can actually generate power in).
Turnover however can be constantly worked on by consciously addressing it. Good turnover rates fall between 90 revolutions per minute (RPM). Time yourself for a minute and count how many times one foot (left or right) touches the ground and you will have your number.