Endurance describes your ability to sustain exercise for an extended time. It’s another way to describe your resistance to fatigue. We usually talk about endurance in reference to aerobic activity that lasts at least 30 minutes, but usually between an hour to half a day. Shorter, more intense workouts are more about anaerobic power. Longer ones we describe as ultra-endurance.

There are three basic components to endurance:

1. Your maximum aerobic capacity.

This describes the highest intensity level you can sustain using your body’s aerobic (oxygen-burning) system. Traditionally, it’s been assessed by measuring your VO2 Max, which is the maximum rate of oxygen your body can consume relative to your weight (ml. oxygen per kg bodyweight per minute). More often these days, it’s assessed by estimating your lactate threshold, which is the percentage of your maximum heart rate above which waste products accumulate and your muscles start burning, but below which you can sustain an effort for a long time.

It’s called “lactate threshold” because we used to believe the burn was caused by an accumulation of lactic acid in the muscles. Now we know that lactic acid is actually recyclable by your muscles as fuel, and does not cause burning. Scientists do not have a perfect understanding of what happens above this magic threshold, but we all know that it exists.

We know that above the threshold, you’re essentially doing an interval workout, and so you’re running on borrowed time. You’ll have to slow down or stop very soon. Below the threshold, you can keep going. But the closer you are to the threshold, the sooner you’ll get fatigued. The benefit of a high lactate threshold for endurance athletes is that it lets you maintain a relatively high intensity while staying below your lactate threshold. Preferably, while staying far below it. Which means you can go fast, and still keep fatigue at bay.

2. Your fat-burning efficiency.

When your body does aerobic work, your muscles burn a mix of sugar (glucose) and stored fat. The body generally prefers glucose, because it’s easier to metabolize. The trouble is, your body has limited supplies of stored sugar. You store it in your muscles and liver, in a form called glycogen. Most amateur athletes do not store enough glycogen to get through a marathon, or even a half marathon. Running out of glycogen, poetically called “hitting the wall,” must be avoided at all costs. This is the point at which your body wants to collapse, and all desire to be conscious, much less run another step, flies out the window. You must conserve your glycogen stores.

And so you turn to the next source of glucose: food or energy drinks. There are limits with these as well. The more fit you are, the less likely that your stomach can release calories fast enough to keep up with your needs.

This is why metabolizing fat is so important. Even if you’re very lean, you probably store enough fat to run a long, long way. A single pound of fat stores 3300 Calories-enough fuel to carry most runners more than 30 miles. The challenge is that bodies burn fat inefficiently. Endurance training can improve this efficiency, encouraging the body to burn a higher percentage of fat, conserving those glycogen stores for longer distances.

3. Your general resistance to fatigue.

With endurance training, your body just gets better at keeping going. All else being equal, you’ll go longer before fatigue sets in. Much of this has to do with changes in your muscles that happen at the cellular level. Much of this process is poorly understood. It’s all good news.

How to Train for Endurance?

The first step is to train your maximum capacity. You need to do a significant portion of your training at much higher intensities than an endurance rate. At least one day a week, and as often as two days, you should do high intensity interval training. This is to boost #1, above: your maximum aerobic capacity.

To boost #2 and #3, you’re going to have to do endurance training, also known as LSD (Long Slow Distance) training. These are steady-state runs, usually at 65 to 75% your maximum heart rate-an intensity level that should allow you to carry on a normal conversation.

Gradually increase the length of these endurance runs, paying attention to intensity level and time, not distance. Ideally, by the end of your race preparation training, your endurance runs will be as long, measured by time, as the race you’re training for. For shorter events, you might increase the length of your final endurance runs to 25% or 50% longer than the race.

To summarize the basic strategy, you need two general types of training to most effectively build endurance: high intensity training to raise your lactate threshold, and long slow distance training to build staying power.

The Importance of Breathing

A less-known factor in endurance is breath control. Ideally you want to breathe using your diaphragm muscles. You’ll know you’re doing it right when your belly expands on each inhale, but your ribcage stays mostly stationary.

Generally, you’ll want to take about 50% more time to inhale as to exhale, with most of your attention placed on the exhale. Inhale through your nose as much as possible. At lower activity levels, you may be able to inhale completely through your nose; with greater exertion you’ll have to inhale through both nose and mouth, with most air entering your mouth. Exhale forcefully, entirely through your mouth.

Many runners find it beneficial to establish a breathing tempo that’s in sync with their running cadence. For example, inhale for three steps, exhale for 2 steps. There’s no set formula; you’ll have to experiment and find what works best at every intensity level. You may find you already do this unconsciously

In addition to being a tool that can improve your physical and mental performance, breathing is a useful informal indicator of intensity. If you’re not working with a heart rate monitor, your breath can be useful to calibrate your training sessions.

Here are the standard training intensity zones:

Easy Zone (for recovery and warmup)

- Just slightly elevated breathing: you can easily sustain a conversation, or even sing.

- Generally corresponds with 60–70% maximum heart rate.

Moderate Zone (Long Slow Distance Training, marathon pace to 20% slower)

- More elevated breathing: you can hold a conversation with short sentences.

- Generally corresponds with 75–85% maximum heart rate.

Tempo or Threshold Zone (tempo training, 5K pace or a bit slower)

- Heavy breathing: you can get out a few words at a time.

- Generally corresponds with 88–92% maximum heart rate.

Maximum Zone (interval training)

- Breathing as hard as you can: you can’t talk.

- Generally corresponds with 95 to 100% maximum heart rate. Breathing and heart rate will increase over the course of the interval.

In summary, endurance training takes place mostly at two intensity levels: moderate and high. The most effective LSD workouts are at a lower intensity level what most runners gravitate toward. You may have to actively hold yourself back. Interval training, on the other hand, requires intensity levels that require discipline to achieve. Your goal is to spend most of your time at these extremes, and less time in the “mediocre middle,” where there are fewer training benefits.