Using the strategy of maximizing time in the optimal intensity zone helps you to determine how long your intervals should be, how many to do, and how much recovery to take between efforts. When you start an interval, it takes a minute or so for your oxygen consumption to get up to the optimal range. Short intervals, therefore, have a higher proportion of “wasted” time than longer intervals. If your intervals are too long, however, you will not be able to maintain the optimal intensity range for the entire workout.
Intervals of two to six minutes typically allow runners to accumulate the most time at 95 to 100 percent of VO2 max. For many runners, workouts consisting of a total distance of 6,000 to 8,000 meters of fast running provide an effective balance between providing a strong training stimulus but not requiring too many days to recover. My favorite workouts are six efforts of 1200 meters or six reps of four to five minutes uphill.