I've talked about intervals before. I spotted this new article; there wasn't much in the abstract, so have bought the whole thing and will comment more when I've had a chance to read it.
Exercise & Sport Sciences Reviews. 36(2):58-63, April 2008.
High-intensity interval training (HIT) is a potent time-efficient strategy to induce numerous metabolic adaptations usually associated with traditional endurance training. As little as six sessions of HIT over 2 wk or a total of only approximately 15 min of very intense exercise (~600 kJ), can increase skeletal muscle oxidative capacity and endurance performance and alter metabolic control during aerobic-based exercise.