What Is the Toughest Part of a Marathon? The Truth About Mile 20

Picture this: You are ten miles from the finish line. Your legs feel like lead pipes. Your mind is screaming at you to stop, to walk, to just lie down on the asphalt. This specific moment-usually around mile 20 or 32 kilometers-is widely known as "the wall." But asking what the toughest part of a marathon is a long-distance running race of 26.2 miles (42.195 km) isn't just about physical pain. It is a complex mix of glycogen depletion, psychological fatigue, and biomechanical breakdown.

If you are training for your first 26.2-mile event, you need to know exactly where the struggle begins so you can prepare for it. The difficulty shifts dramatically as the race progresses. Early on, it is excitement. In the middle, it is rhythm. At the end, it is survival. Let's break down the three distinct phases of suffering in a marathon and how to manage them.

The Physical Crash: Hitting the Wall

The most famous hurdle in marathon running is metabolic. Your body stores energy in the form of glycogen is stored carbohydrates in muscles and liver used for energy. Think of glycogen as your gas tank. An average runner has enough stored glycogen to run about 18 to 20 miles efficiently. Once you burn through those reserves, your body is forced to switch to burning fat. The problem? Fat burns much slower than carbs. This sudden drop in available fuel causes a drastic slowdown, heavy legs, and brain fog. Runners call this "hitting the wall."

This usually happens between mile 18 and mile 22. If you haven't practiced taking in carbohydrates during your long runs, you will likely hit this wall hard. It is not just tiredness; it is a physiological crisis. Your blood sugar drops, your coordination slips, and every step feels twice as heavy. To avoid this, you must train your gut to absorb nutrition while running. Practice consuming 30 to 60 grams of carbs per hour using gels, chews, or sports drinks during your longest training runs.

The Mental Battle: The Cumulative Stress

While the wall is a physical event, the true "toughest" part for many runners is the mental grind that precedes it. A marathon lasts anywhere from two hours for elites to five or six hours for beginners. Maintaining focus for that long is exhausting. By mile 15, the novelty has worn off. The crowds are thinner. The scenery is blurring. You are left alone with your thoughts.

This is where mental toughness is the ability to remain resilient and focused under stress becomes the deciding factor. Many runners fail not because their legs gave out, but because their minds quit. They start calculating how much pain is left instead of focusing on the next stride. This cognitive load increases fatigue. Research in sports psychology suggests that negative self-talk accelerates perceived exertion. When you tell yourself "I can't do this," your body listens. Conversely, breaking the race into small chunks-like "just get to that water station" or "run until that blue car"-keeps the brain engaged and reduces the feeling of endless distance.

Biomechanical Breakdown: Form Under Fatigue

There is a third, often overlooked aspect of marathon difficulty: form degradation. When you are fresh, your running mechanics are efficient. You land softly, your posture is upright, and your arms swing naturally. As fatigue sets in after 20 miles, your core weakens. Your hips drop. Your stride lengthens inefficiently. You start pounding the pavement instead of rolling over your feet.

This mechanical breakdown leads to sharp pains rather than general tiredness. Shin splints, knee pain, and hip flexor strains often appear in the final five miles because your body is no longer absorbing impact correctly. This is why strength training is non-negotiable in marathon training is a structured program preparing runners for 26.2 miles. Strong glutes and core muscles act as shock absorbers. If they fail, your joints take the hit. Incorporating squats, lunges, and planks into your weekly routine ensures your structure holds up when your energy tanks empty.

How to Prepare for the Hardest Moments

You cannot change the distance of a marathon, but you can change how your body and mind handle it. Preparation is the antidote to suffering. Here is how to build resilience for the toughest parts of the race.

• Simulate Race Conditions: Do not save your best shoes or new gels for race day. Use them in your long runs. If your stomach hurts from a certain gel at mile 16 in training, it will hurt worse at mile 20 in the race.
• Practice Negative Splits: Try running the second half of your long runs slightly faster than the first. This teaches your body to perform when tired and builds confidence that you have something left in the tank.
• Strength Train Twice a Week: Focus on single-leg exercises like Bulgarian split squats. Running is essentially one leg jumping after the other. Stronger legs mean less impact on your joints when you are fatigued.
• Visualize the Struggle: Spend five minutes before bed visualizing mile 22. Imagine the pain, then imagine pushing through it. Mental rehearsal primes your neural pathways for resilience.

Understanding Pacing and Perceived Effort

A major reason runners find the last few miles unbearable is poor pacing early on. Starting too fast borrows energy from the end of the race. This concept is called "anaerobic debt." If you run the first five miles at a pace you can only sustain for ten minutes, you are accumulating lactic acid and depleting glycogen prematurely. By mile 20, you are not just tired; you are physiologically bankrupt.

Use a heart rate monitor or the talk test to gauge effort. You should be able to speak in full sentences during the first half of the marathon. If you are gasping for air, you are going too fast. Consistent pacing makes the hardest part of the marathon manageable. Variable pacing makes it impossible.