What Happens in Your Body When You Run 26.2 Miles - Inside the Marathon Machine

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Today’s newsletter is written by Letty from Marathon Media. Letty hosts both, the Marathon Running Podcast and LettyRuns YouTube Channel.

Have you ever wondered what’s really happening to your body during a marathon—from the very first step to the moment you cross the finish line? In this week’s episode of the Marathon Running Podcast, we go deep into the science behind marathon running, and how your heart, lungs, muscles, brain, and fueling strategies all come together to get you through 26.2 miles.

Whether you’re training for your first marathon or chasing a personal record, understanding your body at this level will give you a smarter, more effective approach to training and racing. Here’s a detailed breakdown of what we cover in the episode:

1. The Heart: Your Engine on Race Day

When you start running, your heart ramps up to meet the demands of your muscles. Each beat pumps oxygen-rich blood to fuel energy production.

• Pacing matters: Going out too fast can spike your heart rate too early, leading to early fatigue.

• Heart rate zones: Knowing your aerobic vs. anaerobic zones helps you stay efficient and conserve energy for the later miles.

• Training tip: Include long, steady runs and tempo workouts to strengthen your cardiovascular system and improve endurance.

2. Lungs & Oxygen Delivery

Your lungs are working overtime during a marathon, supplying oxygen and removing carbon dioxide. Efficient breathing helps delay fatigue and supports consistent pacing.

• Breathing strategies: Some runners find rhythm-based breathing (e.g., inhale for 3 steps, exhale for 2) helps maintain pace and focus.

• Altitude and conditioning: Training at slightly higher elevations or incorporating interval work can improve lung efficiency and oxygen utilization.

3. Muscles & Energy Systems

Muscles are where the work really happens. But they don’t just run—they burn fuel, adapt, and signal the brain when they’re tired.

• Glycogen first: Your muscles rely on stored carbohydrates during the early miles.

• Fat as backup: As glycogen depletes, your body switches to fat—slower to convert, which is why pacing and fueling are crucial.

• Fiber types: Slow-twitch fibers handle endurance; fast-twitch fibers are recruited for sprints, hills, and final kicks.

• Training tip: Long runs, interval sessions, and strength work all help recruit the right muscle fibers efficiently.

4. The Brain & Mental Game

Marathons are just as much mental as physical. Your brain regulates effort, focus, and motivation.

• Endorphins & runner’s high: These natural chemicals help reduce perceived pain and boost morale.

• Breaking the race into segments: Mentally focusing on small portions (miles 1–6, 6–13, 13–20, 20–26.2) makes the distance more manageable.

• Visualization & mantras: Practicing these in training makes it easier to push through tough stretches on race day.

5. Fueling & Hydration

Nutrition during a marathon is non-negotiable. Proper fueling keeps energy steady, prevents “the wall,” and supports recovery.

• Carbs & electrolytes: Plan intake based on time and intensity. Common strategies include gels, chews, or sports drinks every 30–45 minutes.

• Hydration: Don’t overdo it—too much water can dilute electrolytes and cause cramping. Balance is key.

• Training tip: Practice your race-day nutrition on long runs to see what works for your body.

6. What Happens After the Finish Line

Crossing the finish line is just the beginning of recovery. Your body needs refueling, rehydration, and gentle care.

• Immediate recovery: Replenish carbs and protein, rehydrate, and walk to loosen muscles.

• Short-term care: Stretch, foam roll, and consider ice baths or compression gear.

• Long-term strategy: Rest, sleep, and plan light workouts in the days after to help muscles repair.

7. Putting It All Together: Training Smarter

Understanding how your body works allows you to train smarter, not just harder. From pacing and breathing to fueling and mental strategies, every piece contributes to a successful race.

• Practical takeaways:

  • Monitor your heart rate to pace efficiently.

  • Train your lungs and muscles with varied workouts.

  • Practice fueling strategies to avoid energy crashes.

  • Build mental resilience with visualization and race-day strategies.

  • Prioritize recovery to prevent injury and improve performance.

Why This Matters:
Marathon running is about more than logging miles—it’s about understanding your body, respecting its limits, and training intelligently. With the right strategies, you can avoid common pitfalls, run stronger, and cross the finish line feeling accomplished rather than exhausted.

🎧 Listen Now: Podcast Link

Pro Tip: On your next long run, pay attention to how your body signals hunger, fatigue, or discomfort. Learning to read these cues is one of the best ways to improve your marathon performance.