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The long run is a staple in almost every runner’s weekly training schedule. Whether you’re training for the 5K or the marathon, at least one day a week is reserved for going long.

Despite the near universal application of the long run, however, many runners don’t know how fast they should be running for optimal benefit. If you run too slow, you won’t produce significant stimulus and adaptation. Run too fast and you run the risk of not being . Making things more difficult, long runs can serve multiple training purposes, each with its own set of intensity and pace recommendations.

How do you determine the optimal pace for your long runs? Let’s look at the science.

What is the purpose of your long run?

The first step to determining the pace of your long run is assessing the purpose and intensity of the run itself. Not all long runs are created equal. Some are designed to simulate marathon conditions or teach you how to finish fast. These types of long runs are considered a hard workout and you should have extra recovery days scheduled after your session to recover accordingly.

On the other hand, some long runs are done at an easier pace and lesser intensity to build aerobic endurance and put “time on your feet.” These types of long runs aren’t exactly recovery runs, but they aren’t designed to be hard, either. If your long run is designed to be a relatively easy day and you run too hard, you’ll start your next workout too fatigued and risk poor performance and injury.

Race-specific long runs in have predefined paces aimed at helping you get more comfortable at running race pace. But what about those “easy” long runs on your schedule? How fast should they be? And why?

Why You Run Long

The optimal pace of a long easy run is determined by the physiological benefits it is intended to induce. Long runs create a number of training adaptations in the body.

Capillary Development

Capillaries, the smallest of the body’s blood vessels, deliver oxygen and nutrients to the muscle tissues. The greater the number of capillaries you have surrounding each muscle fiber, the faster you can shuttle oxygen and carbohydrate into your muscles.

Various studies have shown that capillary development appears to peak at between 60 and 75 percent of 5K pace. This isn’t to say that running really slowly (or much faster) on occasion doesn’t have any benefit. However, running much faster or slower than this pace doesn’t significantly increase or decrease capillary development.

Increased Myoglobin Content

Myoglobin is a special protein in your muscles that binds the oxygen that enters the muscle fiber. When oxygen becomes limited during exercise, myoglobin releases the oxygen to the mitochondria. Simply speaking, the more myoglobin you have in your muscle fibers, the more oxygen you can sequester to the muscle under aerobic duress, like in a race.

While all muscle fibers contain myoglobin, the ones we’re most concerned with targeting during the long run are the Type-I (slow twitch) muscle fibers.  Research has shown that maximum stimulation of Type I muscle fiber occurs at about 63-77 percent of VO2 max. This is about 55-75 percent of 5K race pace.

Increasing Glycogen Storage

The body stores carbohydrates in the muscles for usable energy in the form of glycogen. While this isn’t important for races that last under 90 minutes, when racing the marathon, the more glycogen you can store in your muscles, the longer you can prevent the dreaded bonk.

The goal with easy long runs is to deplete the muscles of their stored glycogen. The body responds to this stimulus by learning to store more glycogen to prevent future depletion.

The faster you run, the greater the percentage of your energy will . While there isn’t any scientific research on the optimal pace that burns significant carbohydrate while still providing enough energy to get through a long run, my experience and studying the training of elite runners has shown that a pace of about 65-75 percent of 5K pace is optimal.

Mitochondria Development

Mitochondria are the microscopic organelle found in your muscle cells that contribute to the production of ATP (energy). In the presence of oxygen, mitochondria break down carbohydrate, fat, and protein into usable energy. Therefore, the more mitochondria you have, and the greater their density, the more energy you can generate during exercise, which will enable you to run faster and longer.

Two researchers, Holloszy () and Dudley () published some of the defining research on optimal distance and pace for mitochondrial development. In short, Holloszy found that maximum mitochondrial development occurred at about 2 hours of running at 50-75 percent of V02max. Likewise, Dudley found that the best strategy for slow-twitch, mitochondria enhancement was running for 90 minutes at 70 to 75 percent V02 max. So what does this mean in real-world terminology?

Summing It Up

The body of evidence is clear: your optimal “easy” long run pace is between 55 and 75 percent of your 5K pace, with the average pace being about 65 percent.

The research shows that running faster than 75% of your 5K pace on your long run doesn’t provide a lot of additional physiological benefit. Pushing the pace beyond 75% of 5K pace only serves to make you more tired and hamper recovery.

In fact, the research indicates that it would be just as advantageous to run slower as it would be to run faster—to an extent. Regardless of your ability level, 50-55 percent of 5K race pace is pretty easy, but the research clearly demonstrates that it still provides near optimal physiological benefits.

If you’re feeling tired and the long run isn’t scheduled to be a “hard” day, don’t be afraid to slow it down. Start on the slower side of the pace recommendations (50% of 5K pace) and slowly pick it up throughout the run if you feel good. The long run is one of the stapes of your training week—make it count!

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Coach Jeff Gaudette has been running for 13 years, at all levels of the sport. He was a two-time Division-I All-American in Cross Country while at Brown University and competed professionally for four years after college. Jeff has competed all over the world, in numerous marathons, and recorded Olympic Trials qualifying times in the marathon and 10,000 meters. He is the Founder, CEO and Head Coach at .