Your body stores roughly 400–600g of glycogen in muscle and 90–120g in the liver — enough to fuel about 90 minutes of hard running. Beyond that, performance deteriorates sharply without exogenous carbohydrate intake. This is the physiological basis of "hitting the wall."

Individual sweat rates range from 0.5 to 2.5 litres per hour — wide enough that generic guidance has limited value. The most reliable protocol is drinking to thirst. Forcing fluids beyond thirst risks hyponatremia (dangerously low blood sodium), which can be more serious than dehydration.

A simple pre/post-run weigh-in reveals your personal sweat rate: 1kg of body weight lost equals approximately 1 litre of sweat. Aim to replace 70–80% during the run, with the remainder post-race.

Sodium is the key electrolyte for endurance runners. It maintains fluid balance, prevents hyponatremia, and helps retain the fluid you drink. Sweat sodium concentration varies considerably between individuals (200–2,000mg/litre), which is why personalised testing (e.g. Precision Hydration's sweat test) is valuable for longer races.

Caffeine is one of the most well-evidenced legal performance enhancers in endurance sport. It reduces perceived effort, sharpens focus and improves time-to-exhaustion. A 2023 meta-analysis (Wang et al.) confirmed significant endurance performance benefits.

For half marathon and marathon, carbohydrate loading in the 48–72 hours before the race increases muscle glycogen by 20–40%. Target approximately 10g of carbohydrate per kg of body weight across the final 48 hours, from familiar low-fibre, low-fat foods. Race morning: eat 2–3 hours before the start, high-carb, low-fibre. A small gel 10–15 minutes before the gun tops up blood glucose as you begin.

The critical asymmetry: uphills cost far more than downhills return. Every 1% of uphill gradient costs approximately 12–15 seconds per mile (7–9 seconds per km), while every 1% of downhill gradient returns only about 8 seconds per mile (5 seconds per km). This means a hilly course with equal gain and loss will always be slower than a flat course of the same distance.

For every 1% increase in grade, energy expenditure increases by approximately 3–4%, meaning a 5% grade requires 15–20% more energy than flat running. Downhills do not simply return this energy — eccentric muscle loading on descents creates fatigue and damage that affects overall performance.

The calculations here are based on the Dill (1965) oxygen cost model and the simplified Minetti (2002) graded running formula — the same underlying science used by Strava and Garmin for Grade Adjusted Pace (GAP). Sources: Runners Connect, TrainingPeaks (McGregor).

The single most important rule: run by effort on uphills, not pace. Trying to maintain flat pace up a hill massively overloads your cardiovascular system and will cost you far more time later in the race than the time "saved."

Controlled downhills are your opportunity. The energy is effectively free — gravity does the work — but you must be careful of quads burning out from eccentric loading. Leaning slightly forward, shortening stride and increasing cadence on descents reduces damage.

On steep uphills above about 15–20% grade, walking is often faster and always more energy-efficient than running. Most elite trail runners walk the steepest climbs.