Why do I bonk at 30k?

What Actually Happens When You Bonk

You're cruising through the marathon. 20km feels great. 25km is manageable. Then at 30km, your legs turn to concrete. Your pace drops. Every step is a negotiation. This is "bonking" - and it's not just mental weakness.

Here's what's actually happening: Your body stores limited glycogen (carbohydrate fuel). At marathon pace, you burn through fuel rapidly. You run out of easily accessible fuel around the 20-30km mark.

The Three Real Causes

1. You Went Out Too Fast

The most common mistake. You felt great for the first half because you were burning through your glycogen stores faster than sustainable. The excitement, the crowd, the adrenaline - they all lie to you about your actual pace.

Real example: In my 2019 marathon (4:35), I went through halfway in roughly 2:01. I ran out of fuel and paid for it from 30km onwards, shuffling to the finish. When I ran my 3:18 PB, I went through halfway in 1:38. I didn't negative split, but because I fuelled well, I held strong while others faded.

2. Your Easy Runs Aren't Easy Enough

If you're running your easy runs too hard during training, you're not building your aerobic system's ability to burn fat for fuel. You become dependent on carbohydrates. Then in the race, you run out.

Most runners run their easy runs 30-60 seconds per kilometer too fast. This matters more than you think.

3. Inadequate Fueling Strategy

If you're not taking in carbohydrates during the race, you're accelerating the bonk. But here's the catch - your gut needs training too. You can't just start taking gels on race day.

How to Fix It

Start Slower Than You Think

For your next marathon, calculate your goal pace. Now subtract 10-15 seconds per kilometre for the first 10km. It will feel easy. That's the point. You're investing in the second half.

Fix Your Easy Runs Now

Use the Training Zones calculator on the homepage. Find your true easy pace. It's probably slower than what you're doing. Run 80% of your weekly kilometers at this pace. Yes, it feels too easy. Do it anyway.

Practice Your Race-Day Fueling

Start taking carbohydrates during your long runs. Aim for 30-60g per hour. Try different gels, drinks, or real food during training - not on race day. Your gut adapts with practice.

The Bottom Line

Bonking isn't inevitable. It's not about toughness or willpower. It's about respecting physiology. Start slower. Train your aerobic system properly. Fuel correctly.

The wall exists. But you don't have to hit it.