How to Avoid Bonking in a Marathon or 70.3

Almost every endurance athlete has felt it.

Heavy legs.
Brain fog.
Pace collapsing.
That slow-motion unraveling at mile 20 or the final 10K of a 70.3.

That’s bonking.

And it’s rarely about “not being tough enough.”

It’s almost always about glycogen depletion, pacing errors, or under-fueling.

Let’s break down how to avoid bonking in a marathon or 70.3.

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What Actually Causes Bonking?

Bonking happens when:

• Muscle glycogen is depleted

• Blood glucose drops

• Carbohydrate intake is too low for intensity

Your body shifts toward fat oxidation — but at race intensity, fat alone cannot sustain pace.

That’s when performance nosedives.

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The 3 Primary Causes of Hitting the Wall

1. Starting Too Fast

Adrenaline lies.

If your first 5 miles feel effortless, you’re probably going too hard.

In both marathons and 70.3 races:

Going out 10–15 seconds per mile too fast can cost you 5–10 minutes late.

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2. Under-Fueling Early

Many athletes wait until they “feel tired” to take their first gel.

That’s too late.

You should begin fueling within:

• 20–30 minutes of race start

• Even earlier in longer 70.3 events

Carbs are preventative, not reactive.

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3. Poor Carb Intake Per Hour

Research consistently supports:

• 60–75g carbs/hour for marathon

• 60–90g carbs/hour for 70.3

Anything below 40g/hour in long races increases bonk risk significantly.

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How to Prevent Bonking in a Marathon

Strategy 1: Pace the First Half Conservatively

Your goal:
Negative split or even split.

If your heart rate spikes early, you’re burning glycogen faster than planned.

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Strategy 2: Start Fueling Early

Example Marathon Plan:

• Gel at 20 min

• Gel every 30 minutes after

• Water at aid stations

• Electrolytes in warm weather

This keeps blood glucose stable.

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Strategy 3: Train Your Gut

Most athletes don’t bonk because they can’t consume carbs.

They bonk because they never practiced consuming enough carbs.

During long runs:

• Practice 60g carbs/hour

• Simulate race intensity

• Test sodium intake

Your gut adapts with training.

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How to Avoid Bonking in a 70.3

Half Ironman is different because the bike leg determines everything.

Fuel on the Bike

The bike is your best opportunity to fuel aggressively.

Target:

• 60–90g carbs/hour

• 500–800mg sodium/hour

If you underfuel on the bike, the run becomes survival mode.

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Don’t Spike Power Early

Overbiking destroys glycogen stores.

Stick to:

• 75–85% FTP for most athletes

• Controlled output

• Smooth cadence

Save ego. Save glycogen.

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Signs You’re About to Bonk

• Sudden chills

• Irritability

• Tunnel vision

• Inability to maintain pace

• Craving sugar intensely

At that point, recovery is slow.

Prevention is easier than correction.

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Final Thoughts

Bonking is not weakness.

It’s mismanaged energy.

To avoid bonking in a marathon or 70.3:

• Pace conservatively early

• Fuel early and consistently

• Train your gut

• Respect carb intake targets

• Manage bike intensity in triathlon

Endurance racing rewards discipline.

Energy management is performance management.