A DOCTOR’S PERSPECTIVE ON RUNNING A MARATHON

Published: 24/04/2026 | By: Priya Gopaldas

What actually happens to your body during a marathon? From the adrenaline-fuelled start to “the wall” and the final push, our ambassador Dr Priya Gopaldas breaks down the science behind every mile – plus the key mistakes runners make and how to avoid them.

The London Marathon is just around the corner and I can feel the nerves slowly building. It’s a funny place to be – part of me is thinking about all the physiology we learnt at medical school, and the other part is remembering exactly how it feels out there on race day, in your legs, your lungs and your head.

A marathon isn’t just about endurance – it’s a full-body stress test, involving your cardiovascular system, muscles, joints and even your gut. That understanding has changed how I approach racing – it’s helped me to pace a bit smarter, fuel a bit better, and recognise when something doesn’t feel quite right.

In this article, I’m hoping to debunk a few marathon misconceptions with medical fact and explain what happens to our bodies at each stage of the race.

COMMON MARATHON RUNNING MISCONCEPTIONS

1. “It’s all about mental toughness.”

Medical fact: Mental resilience matters, but our physiology sets the limits. Once glycogen stores are depleted or hydration is off, no amount of willpower can fully override fatigue.

2. “Drink as much water as possible.”

Medical fact: Overhydration on a hot day can dilute sodium levels, which can be dangerous. Hydration should be balanced – guided by thirst, conditions, genetic predispositions and electrolyte intake. Check out my article on electrolytes for more info.

3. “The hardest part is just the last few miles.”

Medical fact: The final miles are tough because of cumulative physiological stress – glycogen depletion, muscle breakdown and central fatigue. How you run the first half often determines how the last 10 km feels.

4. “Carb-loading is just a big pasta dinner the night before.”

Medical fact: Effective carb-loading happens gradually over 2–4 days before the race. A single large meal the night before is less effective and more likely to cause GI discomfort.

5. “Pain is normal – just push through it.”

Medical fact: Discomfort is expected, but sharp, worsening or one-sided pain isn’t. Pain that alters your running form increases injury risk and shouldn’t be ignored.

6. “Running a marathon is bad for your heart.”

Medical fact: For most healthy individuals, marathon training is safe and beneficial for cardiovascular health. However, it’s still a significant stress, and underlying conditions or warning symptoms (like chest pain or collapse) should always be taken seriously.

MILE-BY-MILE: WHAT’S HAPPENING IN YOUR BODY

Miles 1–5: The Adrenaline Phase

At the start line, adrenaline surges. Your heart rate rises quickly, and everything can feel surprisingly easy. This is where many runners go wrong – starting too fast because it feels effortless.

Your muscles are fresh, but not yet fully warmed into a sustainable rhythm. Early overexertion can lead to premature fatigue later.

My advice:

• Hold back. Run slower than you feel you should (easier said than done, I know!). Start sipping fluids early rather than waiting until you feel thirsty.

Miles 6–10: Settling into Rhythm

Your cardiovascular system begins to stabilise into a steady state. Breathing becomes more controlled, and you settle into your race pace.

At this stage, your body increasingly relies on glycogen (stored carbohydrate) for energy. You’ll feel efficient, but it’s important not to get complacent and to stay on top of your fuelling.

My advice:

• Begin fuelling early (I start from 3 miles, then every 30 minutes). Small, regular carbohydrate intake is key.

Miles 11–15: The Energy Management Phase

This is where the race truly begins. Glycogen stores start to deplete more noticeably, and your body relies heavily on the fuel you’re taking in.

Muscle fatigue may begin subtly (tightness in the calves or heaviness in the quads). Meanwhile, your digestive system is under stress, which is why gastrointestinal issues (nausea, cramping) can appear.

My advice:

• Stick to familiar fuels you’ve practised with
• Combine carbohydrates with electrolytes, especially in warm conditions – I take my gels alongside salt tablets in hot weather.
• Adjust effort slightly if your heart rate or breathing feels harder than expected

Miles 16–20: The Critical Threshold

Often described as the lead-up to “the wall”, this phase reflects declining glycogen stores. Your body begins to rely more on fat metabolism, which is less efficient, meaning your pace may naturally slow. Muscle micro-tears accumulate, increasing soreness and injury risk.

Mentally, this is where doubt often creeps in because the finish line still feels so far away.

My advice:

• Focus on effort, not pace.
• Break the race into smaller segments (this mental strategy is called “chunking” and helps me keep my motivation up).
• Maintain consistent fuelling, even if appetite drops

Miles 21–23: Hitting the Wall (for many runners)

If glycogen stores are significantly depleted, you may experience a sudden drop in energy, commonly known as “the wall”. Coordination can decline, pace slows, and symptoms like dizziness or cramping may develop.

Not every runner hits the wall, though – those who pace conservatively early on and fuel consistently throughout are much more likely to maintain stable energy levels into the later miles.

My advice:

• Adjust your effort, not just your pace
• Continue taking in carbohydrates, even in small amounts
• Stay mentally engaged (this phase often passes with time and fuelling)

Miles 24–26.2: The Final Push

This is the most demanding phase. Fatigue is no longer just muscular – it’s systemic. Your cardiovascular system is under strain, your muscles are exhausted, and even your nervous system is drained.

Emotion often takes over here (the crowd, the finish line, the sense of pushing beyond limits) – use that to drive you across the line. However, it can also make you overlook important warning signs that something may be wrong.

These include:

• Chest pain
• Severe dizziness or collapse
• Confusion or disorientation
• These are not normal and should prompt you to stop and seek medical help.

POST-RACE: WHAT HAPPENS AFTER THE FINISH LINE

After finishing, muscle damage and inflammation peak. This is why soreness often worsens 24–48 hours later. Your immune system may also be temporarily suppressed.

Recovery priorities:

• Rehydrate with fluids and electrolytes
• Refuel with carbohydrates and protein
• Prioritise sleep
• A gradual return to activity is essential – rushing back too quickly increases the risk of injury and burnout.

MY MARATHON PREP ADVICE

If you’re starting your marathon training block, here are some evidence-based tips I wish I’d known earlier:

• Build your mileage gradually – avoid sudden spikes, as these are one of the biggest risk factors for injury
• Follow a structured training plan to balance load, recovery and progression
• Practise your fuelling in training – don’t try anything new on race day (your gut needs training too)
• Stay consistent with hydration and learn what works for you in different conditions
• Include strength work 1–2 times per week, especially for glutes, calves and core
• Don’t skip mobility – small, regular sessions can help reduce stiffness and injury risk
• Choose the right footwear for you – comfort and support matter more than trends

BOTTOM LINE

Training isn’t just about fitness; it’s about preparing your body to handle the huge load of 26.2 miles. It’s a significant physiological challenge – but also incredibly rewarding. With the right understanding, preparation and respect for what your body is doing, you can move through each stage with confidence and reach the finish line feeling strong and in control.