Intensity is Always the Most Potent Producer of Fitness

From Running Research News

(Study comparing runners doing 200 and 400m intervals vs runners doing 30 minute tempo at lactate threshold.)
After 10 weeks, the runners from both groups ran 800-meter and 10-K races. In these competitions, the interval-trained runners fared far better than the tempo-tutored harriers. For example, the interval-based runners improved 800-meter time by an average of 11.2 seconds and bettered previous 10-K times by 2.1 minutes. Meanwhile, the tempo-training devotees shaved just 6.6 seconds from their 800-meter times and upgraded 10-K running by only 1.1 minute, roughly half the improvement achieved by the interval-trained competitors. VO2max soared by 12 percent for the interval runners but nudged upward by only 4 percent for the tempo-trained runners.

These results were observed even though the tempo-trained individuals engaged in a far-greater amount of quality work over the 10-week period. Specifically, the tempo runners completed 58 minutes per week of tempo training, while the interval individuals spent just 31 minutes per week conducting fast interval effort. This led to a 270-minute edge in quality training for the tempo group over the 10-week period.