Interval training programs for runners are very effective for developing an improved capacity to run faster and longer. The long misunderstood value in them is not from the medium to high intensity sessions, but in the balance between those and the low intensity sessions.
Most of the worlds best endurance athletes do no more than 25% of their running, cycling, swimming, rowing or other endurance sport, at high intensity. Those who leave out the required medium to high intensity running miss out on the opportunity to stimulate an improvement in speed of running at aerobic threshold.
This program has a proven track record of getting great results - in the beginner or advanced athlete. There's even flexibility to work these sessions into even marathon training schedules.