From: Stephen Seiler and Espen Tonnesson. See

Endurance training involves manipulation of intensity, duration and frequency of training sessions. The relative impact of short, high-intensity training versus longer, slower distance training has been studied and debated for decades among athletes, coaches and scientists. Currently the popularity pendulum has swung towards high intensity interval training. Many fitness experts, as well as some scientists, now argue that brief high-intensity interval work is the only form of training necessary for performance optimization. Research on the impact of interval and continuous training with untrained to moderately trained subjects does not support the current interval craze, but the evidence does suggest that short intense training bouts and longer continuous exercise sessions should both be a part of effective endurance training. Elite endurance athletes perform 80% or more of their training below their lactate threshold and use high intensity training surprisingly sparingly.