What runners commonly call interval sessions should really be called repeats. Badly reputed and feared, as if they themselves were to blame for something awful and terrible. They are very useful for improving race performance. But unfortunately they are often done too fast, which makes them lose effectiveness and unnecessarily toughens the training.

Interval workouts (or repeats) are nothing more than split training sessions, or pace‑changing workouts, done after a warm‑up of 20’ to 25’ easy jogging. From a physiological and cardiovascular standpoint, intervals are done to improve aerobic power, anaerobic power, and muscular endurance and strength.

In intervals not only are the paces important, recoveries are also very important. Normally, for long‑distance preparation, recoveries should be short, between 30’’ and 2’. Personally, I recommend recovering with a very easy jog to avoid heart rate dropping too much. Recovery should be 30’’ to 45’’ when doing short and very short distances at not‑too‑high paces; it may be 2’ when doing long intervals over 3 km, and 1’ for medium‑distance intervals between 800 and 2000 m.

With this type of training, improvements are faster, more consistent, and more evident than by doing only steady runs.

Running intervals faster does not mean improving more; in fact, quite the opposite — it only leads to performing far below the potential and level at which that runner is training. It is wasted effort, a waste of energy, which often leads to frustration and pain. With this I do not mean that interval workouts do not require effort — of course they do! — but always far from exhaustion and agony.

How should you do them?

The correct paces in intervals depend on each runner’s race times and their objectives. Intensity of pace is the key to any type of training. There is no point in following a training plan to the letter if you fail in the execution of the intervals because they are done too fast or too slow. There is a mistaken belief that intervals must be done flat out from the start. Some elite runners can do that. A recreational runner must think about improving without suffering in intervals.

It is very important to run intervals progressively, as they are better assimilated; joints and tendons suffer less; muscles do not overload as much; the liver works less and more efficiently; recovery is faster and, above all, it reinforces us psychologically. Our body has selective memory, as it remembers from each workout the last thing you do, so by doing just enough and progressively, you advance more safely, faster, and farther with less effort, and especially while enjoying the run. You may ask, how is this done? I will explain it with a simple example. If a runner wants to do 6 repeats of 1000 meters at an average of 4’, what they should do is start around 4’15’’ and finish around 3’45’’ or 3’40’’. If they do so and recover correctly, it means they trained at the right paces. If it was hard to finish at those intensities and recovery is poor, it means those paces were excessive and should be adjusted downward in the next interval sessions. But if they cannot finish in those times, and even run the last two or three slower, it means they were very wrong about the paces they should run. Most likely their true average pace was 5’’ to 10’’ slower than initially planned.

In short intervals those paces would be even faster. In the training plans I indicate pace ranges for doing intervals.

Types of Intervals and Aerobic Capacities

With this type of split training, efforts are made that maximize the oxygen absorption capacity of the muscles. This is only achieved with variable‑pace sessions very close to maximum oxygen consumption (VO2 max).

At slow and moderate running paces, aerobic capacity is developed.

At strong paces, closer to race pace, full development of aerobic power occurs. To achieve this, it is necessary to work with pace changes and long‑distance repeats: from 2,000 m to 5,000 m.

At very strong paces, faster than race pace, anaerobic capacity is developed. This is achieved by training medium‑distance repeats (over 500 m): 500 to 2,000 m. This leads to an improvement in maximal‑effort performance.

Finally, workouts done all‑out (or almost) over very short distances develop anaerobic power. These are the shortest repeats, from 200 to 500 m. They help improve the ability to run with oxygen debt and to tolerate the fatiguing effects of lactate (very important in marathons and half marathons).

In the following table you can see it summarized:

Table to Determine the Type of Training

INTENSITY	DEVELOPS	TRAINING
Easy	Aerobic Capacity	Long steady run
Moderate	Aerobic Capacity & Power	Steady runs and controlled pace
Strong	Aerobic Power	Long intervals and long pace changes. From 2,000 to 4,000
Very Strong	Anaerobic Capacity	Medium‑distance intervals and short and medium pace changes. From 500 to 2,000
All‑out	Anaerobic Power	Short‑distance repeat intervals: from 200 to 500

In summary, intervals and repeats allow combining aerobic endurance with muscular endurance and are essential to improve aerobic power and therefore to achieve your personal best.

It is evident that interval training improves many physical qualities, depending on the distance and the intensity at which it is performed. It helps the body get used to responding to the different demands that will later be required in the race. It leads to an increase in maximum performance, a remarkable improvement in maximum effort capacity, and a greater ability to tolerate lactic acid levels, as the body becomes accustomed to recycling lactate. Cardiovascular efficiency increases due to the higher oxygen demand, the heart enlarges, and muscle blood perfusion improves.