How to Avoid Bonking on Long Runs
It doesn't arrive dramatically. It starts as a small drop in pace that feels oddly expensive. Then your legs go flat. Focus narrows. Every decision takes more effort than it should. Within a kilometre or two, what was a training run has become a survival exercise — and no amount of willpower changes the physiology behind it.
That's a bonk. And it's almost always preventable.
For most runners, bonking is a fuel problem first. Muscle glycogen stores are limited, especially if you start under-fuelled, run too hard early, or rely on water alone for sessions that need carbohydrate. In hotter conditions, dehydration and electrolyte losses accelerate the whole process. Treating these as separate problems is the first mistake. On long runs, fuel, hydration and pacing work as one system.
"One of the first things I tell athletes who are struggling on long runs is to look at when they start fuelling, not what they're taking. Most are starting 40 minutes too late. By the time you feel like you need a gel, blood glucose has already started to drop. You're not topping up at that point — you're trying to stop a slide. The whole principle is to prevent the problem, not react to it."
— Alistair Brownlee, Two-time Olympic Triathlon Champion and founder of truefuels
What bonking actually is
Bonking is the point where carbohydrate availability drops enough to impair performance sharply. Your body can still produce energy from fat, but not fast enough to support the pace you want to hold. That's why marathon pace can suddenly feel impossible while a slow jog remains just about manageable.
This matters because long runs often sit in an awkward metabolic zone. They're not short enough to coast on poor fuelling and they're not always slow enough to rely primarily on fat oxidation. Add marathon-pace blocks, progression finishes or race-specific sections, and carbohydrate demand rises quickly.
A lot of runners misread this problem. They blame fitness when the real issue is that they asked their body to perform a glycogen-heavy session with no plan for replacing what was being burned.
Before the long run: the preparation window
The long run starts the day before. If you arrive with low glycogen because you under-ate, trained hard the previous evening, or skipped breakfast to avoid feeling heavy, you've already narrowed your margin for error.
For morning long runs: eat enough carbohydrate the night before and again at breakfast. The exact amount depends on body size and session intensity, but the principle is simple — start topped up. A runner heading out for 90 minutes easy has very different needs from someone doing 2 hours with marathon-pace work. The harder or longer the session, the more important it is to start fuelled.
Breakfast should be familiar, digestible and carbohydrate-forward. Low fibre, low fat for sessions longer than 90 minutes. Porridge, toast with jam, banana, white rice, sports drink. The best pre-run meal is the one you can absorb calmly and repeat consistently.
During the run: fuel early, not reactively
The cleanest answer to how to avoid bonking on long runs is this: start fuelling before you feel like you need it.
Long run fuelling protocol by session length
| Session length | First gel timing | Subsequent timing | Carb target (g/hr) | Notes |
|---|---|---|---|---|
| Under 75 min | Not usually needed | — | — | Pre-run meal sufficient if well-fed |
| 75–90 min | 40–45 min | — | 20–30g | One gel typically enough |
| 90 min–2hr | 25–30 min | Every 30 min | 40–60g | 1–2 gels per hour |
| 2–2.5hr (easy pace) | 20–25 min | Every 25–30 min | 50–70g | 2 gels per hour |
| 2.5hr+ or with quality | 20 min | Every 20–25 min | 60–90g | Treat like race fuelling |
| Marathon-pace blocks | 15–20 min | Every 20 min | 70–90g | Don't reduce intake at higher effort |
The 40g per truefuels gel makes interval maths simple. Two gels per hour delivers 80g — within the 60–90g range where performance benefits are consistently demonstrated. The 1:1 glucose-to-fructose ratio activates both SGLT1 and GLUT5 transporters simultaneously, which means higher absorption without the gut overload you'd get from glucose-only gels at the same intake rate.
Hydration and sodium: part of the same problem
Many bonks are not pure glycogen failure. They're mixed failures — low fluid intake, sodium losses and rising core temperature making a fuel issue hit earlier and harder.
Replacing fluid with only water can leave you under-hydrated and sodium-depleted at the same time. That's why finishing long runs with heavy salt marks on kit, or feeling bloated despite drinking regularly, are signs the hydration plan needs work — not just the carbohydrate plan.
For long runs in warm weather or with quality work, the truefuels 40/1.0 gel (386mg sodium) provides meaningful sodium delivery alongside carbohydrate. In cooler conditions, the 40/0.25 (98mg sodium) covers lighter needs. Stack both variants — two 40/0.25 and one 40/1.0 per hour — to hit precise sodium targets based on conditions and your own sweat profile. This is what the modular design was built for.
Pacing errors create a nutrition problem
You can eat correctly and still bonk if your pacing is reckless. Starting too hard burns carbohydrate faster, raises core temperature earlier and makes drinking and fuelling less comfortable. That combination is expensive.
This is especially common in social long runs, progression sessions that progress too soon, and race-pace workouts done on tired legs. Intensity changes your fuelling requirement. If intake stays constant while effort rises, the gap grows.
Think of pace as a multiplier. The faster you run relative to your fitness, the more precise the rest of the plan needs to be.
Train your gut like you train your legs
One of the main reasons runners underfuel is fear of GI distress. It's a fair concern, but the solution is practice, not avoidance.
Your gut adapts to carbohydrate intake during exercise. Consistently taking in fuel on training runs — especially at the rates you intend to use in races — improves both absorption capacity and tolerance. If you save fuelling for race day, you're asking for precision from a system you've never trained.
Start with manageable doses and build over several weeks. Record your exact protocol: product, dose, timing, weather, symptoms. Treat it as performance data. A runner consistently tolerating 70g per hour in training is in a far stronger position than one aiming for 90g on race day with no track record.
A practical long-run protocol
For a standard long run of 90 minutes to 2.5 hours: start well-fed, take carbohydrate from 20–30 minutes in, maintain even spacing, and choose sodium concentration based on the weather.
For a quality long run — marathon-pace blocks in the final third, any kind of race rehearsal: treat the fuelling like race nutrition. The session demands are closer to racing than training, and the preparation should match.
The best long runs feel controlled for longer. You stay clear-headed, stride holds together, and the final third becomes a test of fitness rather than a negotiation with low energy. That's what proper fuelling protects. Not effort. Not toughness. Just the ability for your training to actually show up on the day.
