Interval training is the oldest, most studied, and most proven of all performance-boosting training methods for endurance athletes. First you get in shape, then you use intervals to get faster. Intervals were invented for middle-distance and distance track runners, where they worked so well that they soon spread to other sports like swimming and rowing.

Intervals can improve your race times in dramatic fashion, but also require a balanced, judicious approach. Too much interval training leads to burnout and slower races. I often tell runners they should act as if intervals carry a warning label: “Handle with care.”

Interval training was born in Germany in the late 1930s when a coach-physiologist named Woldemar Gerschler teamed with a cardiologist named Herbert Reindel to produce the first systematic training system. They based their program on heart rates, and also on the four variables of track training: distance, speed, number of repetitions, and the rest interval between repetitions.

Today most runners use the word interval to describe the fast part of their workout. Originally, however, Gerschler and Reindel used it for the rest interval between repetitions. They considered it essential to allow for appropriate heart recovery between fast runs.

In 1939 a German runner, Rudolf Harbig, coached by Gerschler and Reindel, smashed the 800-meter world record by a huge margin, 1.6 seconds. When the interval-trained Emil Zátopek won the 5,000 meters, 10,000 meters, and marathon at the 1952 Olympic Games, the interval-training method gained even stronger footing.

Today the effectiveness of interval training is universally accepted, although the practice is not universally adored. When I was in high school in the mid-1960s, all coaches wanted their athletes to mimic Jim Ryun’s heavy interval program. However, I despised intervals on the track, and felt that the workouts made me weaker, not stronger. I responded much better to LSD (long slow distance) training.

Years later I learned how to do interval workouts appropriate to my ability and goals. I’ve continued ever since. Interval training produces great results. You just have to learn how to apply it. And also to stop after six to eight weeks, before you get burned out.

  

Run classic intervals: According to modern interpretation, confirmed by many studies, the most effective time and pace for interval training is 4 to 5 minutes at close to your VO2 max pace—the pace at which you are using 100 percent of your aerobic potential. If you run any faster, you will edge into oxygen debt. In other words, this is the workout that will give you the biggest boost. You don’t need to be treadmill tested in a lab to find your VO2 max pace. It’s essentially the same as your 2-mile race pace.

With that information you can put together a classic interval workout with several repetitions of 800 to 1,200 meters (4 to 5 minutes) at your 2-mile pace. Remember: Don’t overdo it. This isn’t the kind of workout you do every day, and faster isn’t better. I recommend repeating this workout no more than once a week for four to six weeks.

  

Run intervals of different lengths: The best way to avoid staleness and burnout from interval training is to run a variety of quite different intervals. For example, you could run 3200s one week and 200s the next week. In the first case you’d do just a couple of repeats at a pace considerably slower than that of the preceding classic workout. In fact, you might run at your half-marathon pace rather than your 2-mile pace. The next week you could switch to six to eight 200s at a considerably faster pace—roughly your 1-mile race pace. That’s the beauty of intervals—you can use them to hone every part of your running fitness, from your endurance to your final sprint. When combined with a 1- or 2-mile warm-up, and a similar cooldown, intervals make a complete workout.

  

To peak, run fewer, shorter, faster intervals: When top runners want to peak for their best races of the year, they begin to “periodize” their interval training. In other words, they decrease their total running to increase recovery and freshness while also doing shorter, faster intervals. The goal is to rest, build strength, and build speed simultaneously. Each part of the program reinforces the others. With intervals, you can turn this into a science.

When you’re approaching top fitness, it’s sometimes smarter to skip a workout than to nail it. Young runners often can’t appreciate this approach. Veterans understand it through their rearview mirror. “When you’re peaking, a day off can be better than a workout,” says Meb Keflezighi. “We need to be less a type A personality.”