Endurance athletes are constantly looking to improve their fitness and efficiency so, come race day, they can go faster for longer periods of time. Low-intensity workouts—exercises that keep you at a relatively low heart rate for a prolonged period of time—are the key to building that endurance.

Consider elite runners from Kenya, who dominate the sport on a global level. When they run a marathon, they can clock in an average pace of sub-5-minute miles across the 26.2 miles. But when they are training, they will slow it down by two or three minutes a mile. While this approach may seem counterintuitive, running more slowly can ultimately help you run faster.

The 80/20 Rule

Nutrition, strength and genetics all contribute to athletic performance, but many elite endurance athletes use the 80/20 rule to guide their training plans. Eighty percent of their training is spent in a low-intensity heart-rate zone. The other 20 percent of training time is spent performing high-intensity and speed work, interval training and other activities to get the heart rate soaring.

Spending 80 percent of your training at a slow pace, whether biking, running, cross-country skiing or doing other endurance sports, may seem difficult. It may not even feel like a workout when you’re barely breaking a sweat. But research suggests that it will help you get faster.

In 2014, exercise scientist Stephen Seiler, who brought the 80/20 rule to the masses after studying elite athletes in the early 2000s, found that a sample of triathletes who did about 68 percent of their training at a low intensity performed worse than those who followed the 80/20 rule. Seiler observed that training at a lower intensity allows an athlete to endure more volume, which yields better fitness results and, ultimately, improved performance on race day.

Creating a Plan

Calculating your time spent training at a low intensity is easy once you know how much total time you will spend training. But first, you must find your individual heart-rate zones to shape your training. Zones 1 and 2 are considered low intensity, while Zones 4 and 5 are high intensity. Heart-rate tests will give you the information you need to break down your zones.

Once you have your zones dialed in, the 80/20 rule will determine how much you will need to stay in the easy zones to work on increasing aerobic endurance, and how much intense work you will need to do in the higher zones to increase your VO₂max and lactate threshold.

The hard work comes when you want to go faster, but your plan wants you to go slowly. Keep a close eye on your heart-rate monitor to make sure you stay in the correct zones.

What Should it Feel Like?

Exercising at a low intensity should not feel normal. It’s not your regular pace with slightly elevated breathing where you feel strong, fast and capable of going for quite some time. If you think you’re going slowly compared to your normal pace, you still might need to slow it down even more.

If you’re not used going slowly, it will likely feel clumsy and awkward. It will also feel incredibly easy. And that’s a good thing.

Adding in Intensity

There’s still a place for high-intensity workouts in the 80/20 rule. Twenty percent of the total time spent training can be in the higher zones. These intense workouts will get your heart pumping and make you feel like you have worked hard. Some of these workouts should be so hard that you’re going at your max effort for short amounts of time.

Balancing the majority of low-intensity workouts with high-intensity workouts will give you a well-rounded training plan to improve both overall fitness and endurance.