1. Vary Intensities.
Running fitness is determined primarily by the combined volume and intensity of running you do each week and secondarily by the challenge level of your hardest individual runs. Because of this second factor, a week of training in which hard and easy runs are alternated will make you fitter than a week of equal combined volume and intensity in which every run is moderately challenging.
Your typical week should include three designated hard runs: a run featuring short, fast effort; a longer run at a moderately high intensity; and an even longer endurance run. These three hard runs should be separated by slow, easy runs. For example, you might choose to perform your hard runs on Tuesday, Thursday, and Sunday, take Monday off, and do easy runs on Wednesday, Friday, and Saturday.