It’s amazing how few runners ever run at full speed—I mean full speed, the way you ran virtually every time you ran as a kid. All-out sprinting is not only exhilarating, as every six-year-old knows, but it is also great training, even for distance runners. Sprint training increases stride power and running economy, benefits that transfer all the way down to marathon speed and below. Sprinting also forces your body to confront the technical limiters in your stride, resulting in better form.
A little sprinting goes a long way. Elite coach Brad Hudson has his athletes perform a set of 4-12 steep hill sprints of 8-10 seconds once a week after an easy run. The steep hill serves to reduce the risk of hamstring and Achilles tendon injuries.
This is about all the sprinting you ever need to do, except at times when increasing your raw speed is a major fitness priority, when you will also want to perform repeated flat sprints of 100 to 300 meters, also once a week. Start with shorter sprints and extend them gradually over several weeks in order to first increase your maximum speed and then blend your new speed with endurance.