If you are training for a marathon or pushing your long runs beyond 20 miles, you may have experienced something frustrating.
The run starts smoothly. Your stride feels relaxed and efficient. Your breathing settles into rhythm and everything feels under control.
Then somewhere after mile 18 or mile 20, something changes.
Your hips begin tightening & knee starts aching.
Suddenly the run becomes less about performance and more about simply finishing.
Many runners assume this happens because they are tired (and sometimes it really is) or because the distance itself is just too much for them. But the real reason injuries often appear during long runs has less to do with mileage and more to do with what happens to your body as fatigue accumulates. This is called “durability” or “fatigue resistance”.
Understanding these changes is one of the most important steps toward decreasing injury risk and staying healthy throughout marathon training.
In this blog, we will explain why pain often shows up once runners pass the 20 mile mark and what runners can do to reduce their risk of injury.
Why the 20 Mile Mark Is Where Many Running Injuries Start
The 20 mile mark is often treated as a major milestone in marathon training. It represents a point where both physical fatigue and mechanical fatigue begin to significantly influence the way the body moves.
Running form is not something that remains perfectly consistent for the entire duration of a long run. Early in a run, your muscles are fresh and your body can maintain efficient movement patterns. Your stride feels balanced and the forces created with each step are distributed efficiently through the muscles and you’re producing force well as you push off.
As the run continues, fatigue begins affecting the muscles responsible for stabilizing, absorbing shock and propelling the body forward.
When these muscles start to tire, subtle movement changes begin to occur. This is a problem within fatigue resilience and durability.
Examples are a runner may lose some hip extension using the lower back instead due to decreased power in the push off. Shock absorption strategies decrease causing less knee bend. The stride may lengthen out becoming less efficient. Individually these changes seem small, but when repeated thousands of times they can cause stress intolerances in the knees, hips etc. These stress intolerance locations are where runners develop “injury”.
This is why many runners feel strong during the first half of a long run but begin experiencing discomfort later in the effort. The body is still capable of running, but it is no longer distributing and creating forces as efficiently as it did earlier in the run.
What Happens to Your Body During Long Runs
Running places a large amount of repetitive stress on the body. With each step, forces can reach two to three times a runner’s body weight.
During a long run, this adds up quickly.
Once runners push their mileage beyond 20 miles, the body begins entering deeper levels of muscular fatigue as they’re at the top end of their built up capacity. The muscles that normally absorb impact and generate propulsion gradually lose their ability to maintain the same level of performance.
The calve, achilles and glutes play a major role in propulsion forward. As these muscles fatigue, other muscles begin to compensate or the muscles above may be doing their job, but begin to get tight because they’re not strong enough to complete the task at hand.
At the same time, the body’s elastic energy system becomes less efficient. Tendons, like the Achilles tendon, act like springs during running. Tendons release energy with each step forward and replenishes it’s spring as the surrounding joint bends and absorbs force. As fatigue builds, this spring-like system becomes less effective.
In real life, this might look like the calves getting tight at mile 24 because the achilles tendon isn’t storing and producing enough springy forces to propel the runner forward in addition to the glutes not contributing to hip extension as well as they should be. Oftentimes it’s never one muscle that’s causing the problem, it’s a system that is not working efficiently together.
When this happens, the body must work harder to maintain the same pace and efficiency and overtime this additional stress begins to accumulate in specific sites such as the knees, calves or hips.
Running Pain After 20 Miles What It Means
Pain that appears late in a run is often a signal that certain tissues are being asked to handle more load than they are prepared for.
For many runners, knee pain is the first issue that appears. This often occurs when the muscles responsible for shock absorption begin to fatigue. When eccentric quad capacity decreases, forces are less efficiently absorbed placing more force on structures around the knee or the muscle itself. Over time, this repeated stress can irritate structures causing “injury”.
Hip discomfort can also develop during long runs when the muscles responsible for pelvic control begin struggling to keep up with the demands of the run. When pelvic stability decreases, the hip joint absorbs more rotational stress than usual. This can irritate surrounding muscles and tendons.
Hamstring tightness is another issue runners frequently notice during longer runs. Many runners believe this means they need more stretching, but in many cases the underlying issue is a stress intolerance causing fatigue. When the posterior chain (glutes and calves) are no longer producing force efficiently in push off, the runner compensates by pulling themselves forward with the hamstrings. Over time this increased workload can lead to persistent tightness or even strain. The goal is that the hamstrings are strong enough to tolerate this type of compensation because it is normal with fatigue, while also increasing the fatigue resistance of the posterior chain to decrease the frequency at which this fatigue occurs.
The key point is that pain during long runs is rarely random. It is often the body’s way of signaling that fatigue has begun changing how forces are distributed through the body.
Early Warning Signs Runners Should Not Ignore
Long runs rarely cause injuries without warning. Most runners notice early signals that something is beginning to overload before a full injury develops.
Some of the most common warning signs include:
• Pain that consistently appears after a certain mileage point
• Tightness in a localized area that increases as the run continues
• Difficulty maintaining normal running form late in the run
• Discomfort that lingers after the workout is finished
• A noticeable change in stride or cadence when fatigue sets in
Recognizing these warning signs early allows runners to adjust training, address strength deficits, and prevent small problems from becoming injuries that prevent you from running.
The Biggest Mistake Runners Make When Training for Long Runs
One of the most common mistakes runners make when preparing for longer distances is believing that improving endurance simply requires running more miles.
Mileage plays an important role in marathon training, in fact lower overall weekly mileage in a training block is linked to higher injury risk as mileage is what prepares a runner for the race they’re training for. That said, it is not the only factor that determines whether a runner stays healthy.
Running long distances requires durability and fatigue resistance. It requires the body to repeatedly absorb impact while maintaining efficient movement patterns. If the muscles responsible for stabilizing the body, absorbing force and producing force are not strong enough to support this workload, mechanical breakdown past mile 20 is more likely as the body fatigues leading to areas of potential stress intolerance and thruster injury.
Successful long distance runners often focus on supporting their running with strength training to improve fatigue resistance so this breakdown is less likely to happen and to increase their overall capacity.
Another key factor is allowing the body time to gradually adapt to increasing mileage. Tendons and muscles require time to build tolerance to repetitive stress. Increasing the “long run” distance within a training block too quickly (more than 10%) can overwhelm this adaptation process and lead to irritation or injury.
Additionally, it is difficult for the body to adapt and stay resilient if energy lost is not replenished. This means fueling during your race and even after. New research suggests that runs longer than 2hrs require 60-90g of carbs per hour in order for muscles to replenish their energy stores and to have the fuel to recover after the long run. This is 2-4 gels per hour depending on the gels you take!
Running Beyond 20 Miles Without Breaking Down
Running beyond 20 miles is one of the most challenging parts of marathon preparation. It requires not only cardiovascular endurance but also the strength and resilience to maintain efficient movement for an extended period of time.
When runners focus only on accumulating mileage, they often overlook the physical systems that allow the body to handle the demands of distance running as well as the recovery.
But when a strength base is built first the body becomes far more resilient so the 20 mile mark becomes simply another step toward the finish line, rather than a point where things fall apart.
For runners training for a marathon or pushing their longest runs further than ever before, understanding how fatigue and fatigue resistance affects the body can make the difference between recurring injuries and consistent progress.
With the right preparation, runners can push beyond 20 miles feeling strong, efficient, and confident that their body is ready for the distance ahead.
