In brief:Successful long-distance running is often attributed to a high VO2max, but running economy—the steady-state oxygen consumption at a given speed—may be more important. The physiological changes in a 31-year-old elite runner were studied during an 18-week training program using both interval and endurance running. Most improvements in running economy were noted during or immediately after weeks of increased interval training. Among runners of similar ability, a small difference in economy can make a large difference in finishing time in races longer than 10,000 meters.