How to Fuel Long Training Runs Properly
The long run is not just a fitness session. It is the place where marathon performance is either built or quietly undermined — and the difference often has nothing to do with the legs. An athlete who runs 30km of quality work every Sunday for sixteen weeks, but consistently does it at 30g of carbohydrate per hour, has trained a body that can sustain marathon fitness but not marathon fuelling. On race day, when the plan calls for 70g/hr, the gut has never seen it. The first hour goes fine. The second hour starts to strain. By kilometre 32, the session they trained for becomes something different entirely.
This is the specific gap that ruins well-trained athletes in marathons. Not a lack of miles. Not insufficient threshold work. A fuelling protocol that was never rehearsed at the volume the race demands.
Knowing how to fuel long training runs properly means treating the nutritional side of each long session with the same deliberateness as the pacing structure. The target isn't to survive the run. It's to complete it in a state that confirms the protocol is ready.
"A long run is the best laboratory in endurance sport. The intensity is controlled enough to test variables clearly, the duration is long enough to expose problems that short sessions hide, and you can reset the protocol from one week to the next without the stakes of a race. I've always used long runs to build and test fuelling plans, not just fitness. If the nutrition doesn't hold up in training, it won't hold up on race day. That's not pessimism — it's the most useful thing about long runs."
— Alistair Brownlee, Two-time Olympic Triathlon Champion and founder of truefuels
Setting carbohydrate targets before you leave
The most reliable way to underfuel a long run is to start with no specific target and rely on how you feel. Appetite is suppressed during exercise. Hunger signals are unreliable at pace. And by the time a carbohydrate deficit is noticeable — hollow legs, rising perceived effort, concentration dropping — performance has already started to decline.
Set a carbohydrate target per hour before the session begins. Let the session type determine it.
| Long run type | Duration | Carb target (g/hr) | Sodium approach | Key mistake to avoid |
|---|---|---|---|---|
| Easy long run | 90 min–2hr | 30–50g | Gel built-in electrolytes | Starting too late (>40 min) |
| Progressive long run | 2–2.5hr | 50–70g | 40/0.25 gels + water | Treating it like an easy session |
| Marathon-pace long run | 2–3hr | 60–80g | 40/1.0 gels or gel + sachet | Racing the early miles; under-fuelling the quality portion |
| Hot-weather long run | Any | Upper range + sodium | Pre-load + 1 sachet/hr | Treating heat as a cool-day problem |
| Race-rehearsal long run | 2.5–3.5hr | Match race target | Full race-day protocol | Using a different product than race day |
| Back-to-back long days | 2hr each | 60–80g; same next day | Post-day-1 recovery sachet critical | Neglecting day-one recovery as day-two fuel |
Pre-run preparation: the hour before matters
The session starts with the meal before it. Liver glycogen drops overnight, which means morning long runs should always include a pre-run meal — not a large breakfast, but enough to restore baseline glycogen and start the run without a carbohydrate deficit already in place.
Two to three hours before a long run: familiar, easily digested carbohydrate. Porridge, white toast, a bagel, banana, rice. Low fibre, low fat. The specific food matters far less than the timing and quantity. If you've been using the same pre-run breakfast for six months, continue using it. Novel foods close to a key session introduce variables that don't help.
For early morning sessions with a compressed timeline: reduce the meal size, reduce the fat and fibre content, and move the eating window as close to 60 minutes pre-run as your stomach will tolerate. A small, easily digested carbohydrate meal is better than nothing or a large meal too close to the start.
In the 10–15 minutes before starting, a final gel top-up maintains blood glucose into the first phase of the session. For marathon-pace blocks in particular, beginning a hard effort with topped-up glycogen rather than chasing it from behind makes a visible difference to the first 20 minutes.
Fuelling during the run: start earlier than feels necessary
The time between taking carbohydrate and it becoming available in the bloodstream is 15–20 minutes. Start fuelling before you feel you need it. For most long runs, that means taking the first gel within 20–30 minutes of starting — before the legs have given any signal that energy is running low.
Continue at consistent intervals. Every 20–25 minutes is a reliable structure for most athletes, delivering steady carbohydrate rather than reactive bursts. One truefuels gel (40g carbohydrate, 1:1 glucose-fructose ratio) every 25 minutes gives approximately 96g per hour — above the threshold where the GLUT5 fructose transporter begins meaningfully contributing alongside SGLT1. For athletes working towards that ceiling, this is the mechanism that makes high hourly intake work: two independent absorption pathways active simultaneously rather than one being overwhelmed.
For athletes new to this intake level, start lower — 40–60g/hr — and build across weekly long runs. The gut adapts to higher carbohydrate intake the same way the legs adapt to higher mileage: progressively, with time.
truefuels Training Bundle
20 gels and 20 electrolytes — the fuel to rehearse your full race protocol across a training block, so race day becomes execution, not experiment.
See the bundle →Hydration and sodium: not background noise
Fluid and sodium deserve the same deliberateness as carbohydrate on long runs. The three systems — fuel, fluid, electrolytes — interact, and a strong carbohydrate plan that's paired with insufficient sodium will produce GI problems that get blamed on the gel rather than the protocol.
Fluid: 400–700ml per hour in mild conditions, adjusting upward in heat. Weigh before and after a representative session without drinking to estimate your sweat rate. Use that number to set a realistic hourly fluid target rather than drinking to vague advice about "drinking when thirsty."
Sodium: The truefuels 40/0.25 gel delivers 98mg sodium per serving. The 40/1.0 gel delivers 386mg. For cool conditions and moderate sweat rates, the gel's built-in sodium often covers most of the hourly need. For warm sessions, marathon-pace work, or athletes with high sweat sodium losses, adding an Active Hydration sachet (390mg per 500ml) provides the additional layer. The two products are designed to work in combination — the gel's sodium supports carbohydrate absorption, the sachet adds the hydration layer when conditions demand more.
Using long runs to build gut tolerance
This is where most athletes either invest properly or leave a critical performance gap. Gut training — deliberately increasing carbohydrate intake across weekly long sessions over several months — is what allows athletes to reach 80–90g/hr on race day without distress.
The protocol:
1. Identify current sustainable hourly carbohydrate intake on long runs.
2. Increase by 10–15g/hr across 2–3 weeks, testing each level across at least two long sessions before progressing.
3. Test at marathon pace, not just easy pace — gut tolerance at easy pace overstates what's achievable when effort rises.
4. Track finish state. A well-fuelled long run leaves you tired; a poorly fuelled one leaves you bonked, unable to eat for hours, or dealing with late-session nausea.
One race-rehearsal long run per training block should use the exact protocol planned for race day: same products, same timing, same gel variants, same combination of gel and electrolyte sachet. Discover the problems there, not at the 30km mark.
Common fuelling errors on long runs
Starting too late. The runner who feels fine for the first 8km and doesn't take the first gel until 45 minutes in is already behind. Fine early doesn't mean adequately fuelled early.
Training at 30g/hr and racing at 80g/hr. The gut doesn't adapt to higher intake without practice. If long runs consistently underdeliver carbohydrate versus the race-day target, the race is where that gap gets discovered.
Avoiding fuelling because of past GI problems. Most gut distress on long runs is a protocol problem — taking too much too late, using the wrong concentration, combining gels with insufficient fluid. The solution is usually to fix the protocol, not reduce the fuel. Deliberately under-fuelling long runs in response to past GI problems trains neither the gut nor the fuelling habit needed for race performance.
Treating every session the same regardless of conditions. A long run in February and a long run in June in 24°C heat are different physiological problems. The fuel and fluid plan should adapt.
A simple framework
For runs under 75 minutes in cool conditions: pre-run meal, water during the session, recovery nutrition after. For runs over 90 minutes: gel within 30 minutes of starting, continue every 20–25 minutes, sodium matched to conditions. For marathon-pace long runs: use race-day protocol exactly.
Track honestly. Did pace hold in the final third? Did energy feel stable? Did the stomach cooperate? Was recovery the next morning normal? Good answers across all four means the fuelling is working. Bad answers in any one of them means the protocol needs adjusting — and the long run is exactly the right place to find that out.
The fuel to rehearse your race protocol in training.
20 gels and 20 electrolyte sachets — everything needed to build and test your carbohydrate and sodium plan across a training block, so race day becomes execution, not experiment.
See the Training Bundle →Use your long runs to build the plan.
The Fuelling Protocol tool sets the carb and sodium targets to rehearse — so race day is execution, not experiment.
