If you are too fast or too slow, do the same distance the following week. If you hit the pace exactly, extend the distance by another mile the next week. Do not go beyond this, because you risk injury and tiring yourself. Speed work, track sessions and fartlek Training at speed helps to build stronger muscles and improve your running efficiency. Even if you are training for the marathon, speed work will help both your speed and endurance.
Serious athletes do a lot of speed work, and relatively little long distance work.
However, speed work is hard on your body, and it is therefore the training which is most likely to injure you. Most speed work is done at an athletics track. The aim is usually to complete the distance in the same time on each repetition. Work out the speed at which you run a mile. Jog 200m, taking about a minute.