The best distance runners in the world train from 100 to 160 miles per week. Yet, the belief has developed among some runners that high mileage is not necessary for high level running performance. Scientific evidence has even supported this belief-studies show that you can maximize your VO2 max on less than 40 miles per week. So, why does almost every world class distance runner do high mileage? Are they wrong?

VO2 max, which is primarily determined by the size and strength of your heart’s left ventricle, is only 1 piece of the performance puzzle. Performances in races lasting more than 30 minutes are related more closely to adaptations that occur inside your muscles. These positive adaptations continue to occur in your muscles as your mileage increases. According to the principle of specificity of training, however, higher mileage is necessary for a marathoner than for a 10,000 meter runner.

Although racing performance improves with increased mileage, the incremental improvement decreases the more mileage you do. In Daniels’ Running Formula, renowned coach and physiologist Jack Daniels explains the principle of diminishing return, saying , “Adding more and more mileage to your weekly training does not produce equal percentages of improvement in competitive fitness.” Increasing from 60 to 80 miles per week, therefore, will not improve performance as much as increasing from 40 to 60 miles per week, but it may produce a benefit nonetheless.

More is only better to a point, however. Everyone has their own individual current mileage limit which is dictated by your biomechanics, past training, injury history, shoes, running surface, diet, and various other life stressors. The challenge for those pursuing excellence is to find the mileage range that you can handle without breaking down.

What are the benefits of high mileage training?

So, what positive adaptations occur when you increase your mileage? Not many studies have been done in this area because to obtain meaningful results you need to be able to measure what is happening inside the runner’s muscles, which requires repeated muscle biopsies, which are painful! The existing evidence, however, indicates that the following adaptations occur:

Increased capacity to store glycogen (carbohydrate)

In races lasting more than one hour, the ability to store glycogen limits performance. High mileage training repeatedly depletes your body’s carbohydrate stores, which provides a powerful stimulus for your muscles and liver to store more glycogen. This allows you to maintain a challenging pace longer before reaching “the wall.”

Improved ability to utilise fats

High mileage training also improves your ability to utilise fats and thereby spare your carbohydrate stores. As you increase your training volume, you rely more on fat and less on carbohydrates to run at a given speed. As a result, your glycogen stores last longer.

Increased capillary density in your muscles

Capillaries are the smallest blood vessels. They bring oxygen and fuels to your muscle fibers, and remove waste products such as carbon dioxide. High mileage training provides a stimulus to increase the number of capillaries per muscle fiber, which improves the ability of your muscles to produce energy aerobically.

More mitochondria in your muscles

Mitochondria are the aerobic energy factories in your cells. Your performance as a distance runner is closely related to the density of mitochondria in your running muscles. As you increase your mileage, the number and size of the mitochondria in your muscle fibers increases, which allows more fat and less carbohydrate to be used at a given pace. This adaptation further delays glycogen depletion. In a series of studies at the University of California at Berkeley, rats were progressively trained to run. As a result, the amount of mitochondria in their muscles doubled and their running endurance improved by 400%. With more mitochondria in your muscles, you can also produce more energy aerobically, and, therefore, maintain a faster pace without building up lactic acid in your muscles.

Mental toughness

There is also a psychological component to the benefits of high mileage. You gradually push out your horizon for psychological fatigue, which can be particularly useful for races. In 1981, when I was coming up through the marathon ranks, 2:10 marathoner Gary Bjorklund revealed to me that he was running 160 miles per week. When I asked if that much mileage was necessary, he replied, “It’s not necessary before every marathon, but you need to do it at least once to know you can.” While Gary’s mileage would have been excessive for the vast majority of runners, it gave him a boost of confidence. The same confidence boost applies for anyone increasing their mileage.

Now we have seen why increasing your mileage can help your running performance. But, how can you increase your mileage without breaking down through injury of overtraining? Learn how to successfully increase your mileage while minimizing your risk of injury. 

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