Which energy system provides energy for longer-duration, lower-intensity activity such as a marathon?

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For long-duration, low-intensity activity, energy comes primarily from the aerobic system, which uses oxygen to generate ATP in the mitochondria. This process smoothly breaks down carbohydrates and fats to produce a large amount of ATP, enough to sustain hours of work when the effort is steady. The rate of ATP production is slower, but it doesn’t rely on quick fuel stores, so fatigue is pushed back during endurance efforts. This makes it the dominant source during a marathon, where oxygen supply meets the steady energy demand and lactate buildup is minimized. In contrast, the phosphagen system fuels very short, explosive efforts, and the glycolytic system powers higher-intensity work for only a few minutes before fatigue sets in. The oxidative and aerobic descriptions refer to the same oxygen-using process, just viewed from different angles, so the best fit for a marathon is the aerobic (oxidative) system.