Somewhere around 90 minutes into sustained effort, your body's glycogen stores start running low regardless of fitness level, and performance drops off sharply if you haven't been replacing carbohydrate along the way. This is the well-known "wall" — and it's largely a fueling problem, not just a fitness one.
The targets, and why they moved
Older fueling guidance topped out around 30-60 grams of carbohydrate per hour, based on the rate at which a single type of sugar (glucose alone) could be absorbed through the gut. More recent research on multiple-transportable-carbohydrate sources — glucose paired with fructose, which use separate absorption pathways — has pushed the achievable ceiling higher, with well-trained runners using mixed-sugar fueling tolerating up to roughly 90 grams per hour in some cases. That upper range isn't a starting point for most runners; it's a trained capacity built up over weeks of practicing fueling in training, the same way you'd build mileage.
Running the numbers
The Carb Fueling Planner takes your expected duration and target grams-per-hour and returns total carbohydrate needed for the effort.
Worked example: A 4-hour marathon at a conservative 45 g/hour target needs 180 grams of carbohydrate total — roughly four to five standard energy gels (most carry 22-25g each), spaced across the race rather than front- or back-loaded.
The part the math can't plan
Total grams needed is the easy half of this. The harder half is gut tolerance, which has to be trained specifically — race day is the wrong place to test a new gel brand or a higher intake rate than you've practiced. Start conservative (30g/hour) in early long runs, increase gradually over several weeks if tolerated, and treat your eventual race-day number as something you've rehearsed at least half a dozen times in training, not something you calculate the week of the race and hope works.