Energy Cost Of Running
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It's the amount of metabolic energy, often expressed in kilocalories or joules, required to run one kilometer, largely independent of pace within a normal range, since running is close to a fixed energy cost per unit distance rather than per unit time.

Unlike cycling, where air resistance dramatically increases energy needs at higher speeds, running's energy cost per kilometer is fairly stable across a wide range of paces because the biomechanical cost of each stride doesn't change drastically with moderate pace changes. This means running faster mainly changes how quickly you use that fixed energy, not how much total energy the distance requires.

Energy cost per kilometer scales roughly with body weight, since you're moving more mass over the same distance, which is why energy cost is often expressed per kilogram of body weight to allow fair comparison between runners of different sizes.

Multiplying the per-kilometer energy cost (adjusted for your body weight) by the total distance run gives a reasonable estimate of calories burned, which is generally more accurate than calorie estimates based on time and heart rate alone, especially for steady-state running.

Yes, running on sand, trails, or uphill significantly increases energy cost compared to a flat road or track due to added mechanical work and reduced elastic energy return, so a calculation based on flat-road energy cost will underestimate the true energy demand on varied terrain.