To gain the best results for the time spent in training, it is important to run at your best aerobic speed: i.e. at speeds at a level just under your steady state or Maximum Oxygen Uptake. This is theory and can be applied in practice.
Even very slow running will effectively increase general cardiac efficiency and therefore raise the oxygen uptake. However, by running at speeds much below the maximum oxygen uptake level, it is going to take much longer periods of time to gain the same results than if the if the rates of speed were at faster aerobic levels. In other words: one can run too fast or too slow and it is important to control the running efforts as well as possible if the optimum results are to be achieved in the time spent exercising.