The Wall: What Bonking Actually Is and How to Avoid It
Glycogen depletion, the physiology of the thirty-kilometre collapse, and why it feels psychological when it is metabolic. How to fuel so it does not happen, and what to do in the ten minutes after it starts.
- The wall is glycogen depletion. Your muscles run out of their preferred fuel.
- Stored glycogen covers roughly 90 to 120 minutes of hard running. Not four hours.
- It arrives around 30 km because that is when the tank empties at marathon effort.
- Prevention is fuelling from 45 minutes in, before you need it. It cannot be fixed reactively.
- Going out too fast burns glycogen disproportionately and is the most common cause.
There is a specific and unmistakable experience that a great many marathon runners have exactly once before they take fuelling seriously.
Everything is fine. You are on pace, the effort is manageable, you are thinking about the finish. Then over the course of about a kilometre, it stops being fine. Not gradually — the pace falls away from you despite trying harder, your legs feel like they belong to someone else, and your thinking goes strange.
That is glycogen depletion, and it is a fuel problem rather than a fitness one.
What is actually happening
Your body has two main fuels for running: carbohydrate, stored as glycogen in muscle and liver, and fat.
Fat is essentially unlimited — even a very lean athlete carries many hours of it. Carbohydrate is severely limited: roughly 1,500 to 2,000 calories in a trained person, which covers about 90 to 120 minutes of hard running.
The critical constraint is that the harder you run, the more you rely on carbohydrate. At easy aerobic paces you burn a high proportion of fat. At marathon effort, carbohydrate is doing most of the work — and it is the fuel that can run out.
When glycogen is depleted, you are forced onto fat, which cannot be metabolised fast enough to support marathon pace. You do not choose to slow down. You physically cannot maintain the pace, because the energy is not arriving quickly enough.
The mental fog is the same problem in your brain, which runs on glucose and does not appreciate the shortage.
Why 30 kilometres
There is nothing special about the distance. It is arithmetic.
If stored glycogen covers roughly two hours of running at marathon effort, and a typical amateur marathoner is running somewhere between four and five and a half minutes per kilometre, the tank empties somewhere around 28 to 33 km.
Which produces two useful corollaries:
Faster runners hit it later in distance terms — often not at all, because they finish before the tank empties and because they generally fuel well.
Slower runners can hit it earlier in distance but often do not, because they are running at a lower intensity where fat contributes more.
The people most at risk are those running hard for a long time — which describes most people attempting a challenging marathon goal.
Why going out fast is the main cause
This is the part worth internalising, because it is the largest controllable factor.
The proportion of fuel coming from carbohydrate rises steeply with intensity. Running the first half of a marathon 15 seconds per kilometre faster than goal pace does not cost you a proportionally small amount of extra glycogen — it costs disproportionately more, because you have shifted the fuel mix toward carbohydrate for over an hour.
You spend the fuel early that you needed at kilometre 32.
This is why the athletes who blow up are so consistently the ones who felt wonderful at 10 km. They were not wrong that they felt good — they were spending a limited resource while feeling good, which is exactly what makes it hard to notice. The pacing discipline that prevents it is the same argument as in the sub-4 hour marathon plan.
Prevention
Four things, in order of importance.
1. Pace conservatively. More races are saved here than by any fuelling strategy. Even splits or a slight negative split.
2. Start topped up. Raise carbohydrate intake for the two to three days before, which fills muscle and liver glycogen. The light version of carb loading described in what to eat the week of a HYROX race applies equally to a marathon, and buys you a genuinely larger tank.
3. Fuel from 45 minutes, before you need it. 30 to 60 g of carbohydrate per hour, taken on a schedule rather than in response to feeling low. Absorption takes time, so anything you take reactively arrives 15 to 20 minutes too late.
4. Train the gut so you can actually take it in. The most common failure is not a bad plan but an untrained stomach that rejects the plan on race day — the protocol for fixing that is in gut training.
Training your body to need less
Two adaptations reduce your reliance on carbohydrate at a given pace.
Improved fat oxidation. Sustained easy and long running trains your ability to use fat at higher intensities, sparing glycogen. This is one of the main physiological arguments for the long run and for a large easy base.
Better running economy. A more economical runner uses less total energy at a given pace, so the glycogen lasts longer. Heavy strength work contributes here, which is one of the less obvious arguments in the only six lifts a runner needs.
Neither eliminates the ceiling. They raise it.
What to do when it happens
It will still happen occasionally. The response determines whether you lose five minutes or forty.
1. Slow down immediately and substantially. Not slightly — meaningfully. At lower intensity you shift back toward fat metabolism, which you still have plenty of. Fighting it at pace is the single worst option and reliably converts a bad finish into a walk.
2. Take simple carbohydrate straight away. Gel, sports drink, anything fast. Expect 15 to 20 minutes before it helps.
3. Take fluid with it. Concentrated carbohydrate without fluid sits in the gut.
4. Reset the target. The goal time is gone. The finish is not. Athletes who accept the new race lose a few minutes; athletes who keep attacking the old target walk from 35 km.
5. Keep moving. Walk-run beats stopping. Stopping makes restarting far harder, physically and psychologically.
Distinguishing it from other problems
Late in a long race, several things feel similar. The distinctions matter because the fixes differ.
| Signs | Fix | |
|---|---|---|
| Glycogen depletion | Sudden pace collapse, mental fog, ravenous | Carbohydrate, slow down |
| Dehydration | Gradual onset, rising heart rate, headache, nausea | Fluid with sodium |
| Heat illness | Chills, confusion, stopping sweating | Stop. Cool. Get help |
| Ordinary fatigue | Gradual, effort still produces pace | Nothing. Keep going |
The heat row is the one to actually memorise. Confusion and chills in a hot race are not bonking, and treating them as a fuelling problem is dangerous — the warning signs are set out in running in Indian heat.
The reframe
Hitting the wall gets discussed as a test of will, and it is the one thing it is definitely not. You cannot want your way out of an empty fuel tank.
It is a solvable logistics problem: start with more fuel, spend it more slowly, and add fuel while you go. Do those three and most people never meet the wall at all.
The ones who do are almost always the ones who felt great at 10 km and acted on it.
Questions people ask
What does hitting the wall feel like?
An abrupt and disproportionate change — pace collapses despite maximum effort, legs feel heavy and unresponsive, and thinking becomes noticeably foggy because the brain is also short of glucose. It is distinct from ordinary late-race fatigue in how suddenly it arrives and how little effort seems to change the outcome.
Why do runners hit the wall at 20 miles?
Because stored muscle glycogen covers roughly 90 to 120 minutes of running at marathon effort, and for most amateur marathoners that runs out somewhere around 30 to 32 km. The distance is not magic — it is simply where the tank tends to empty at that intensity for that duration.
How do you avoid bonking?
Fuel early and regularly, starting around 45 minutes in and taking 30 to 60 g of carbohydrate per hour, rather than waiting until you feel depleted. Start the race properly fuelled with topped-up glycogen, and pace conservatively — going out too fast burns glycogen disproportionately fast and is the single most common cause.
Can you recover from a bonk mid-race?
Partially, and slowly. Taking carbohydrate helps but takes 15 to 20 minutes to reach the muscle in useful quantities, and you will not return to your original pace. Slow down substantially, take simple sugars and fluid, and accept a revised finish time — continuing to push through it makes the outcome considerably worse.
What is the difference between bonking and dehydration?
Bonking is running out of carbohydrate, producing sudden pace collapse and mental fog. Dehydration develops more gradually with rising heart rate, higher perceived effort and often headache or nausea. They can occur together and can feel similar late in a race, but carbohydrate fixes one and fluid with sodium fixes the other.
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