Running, whether for fitness or competition, can feel especially challenging during the first ten to fifteen minutes. Many runners describe this initial phase as the hardest part of their workout. Breathing often feels labored, legs may feel heavy, and a pace that feels easy later on can seem unexpectedly difficult at the start. This is a common experience across all levels of runners, from beginners to elite athletes. When you begin running, your muscles require a rapid supply of energy. However, the body’s system for producing energy using oxygen—known as aerobic metabolism—cannot respond immediately. To compensate, the heart rate increases, breathing becomes deeper and faster, and blood flow shifts toward the working muscles. These changes help deliver more oxygen to the muscles, which they use to generate energy. While these responses happen quickly, they are not instantaneous. This process of increasing oxygen consumption is called oxygen uptake (VO₂) kinetics. It describes how fast the body can ramp up oxygen use when starting exercise. This transition occurs in three stages. In the first, the body focuses on increasing blood flow and oxygen delivery. In the second, muscles begin using aerobic energy more actively. By the third stage, oxygen delivery and demand are balanced, and the body reaches a steady state. At this point, the body has fully adapted to the demands of running. Before reaching this steady state, muscles still need energy. To bridge the gap, the body temporarily uses stored energy within the muscles. This is why the first few minutes of running can feel so tough—breathing feels more effortful, legs feel sluggish, and the perceived effort is high. Around ten to fifteen minutes into a run, many runners notice a shift. The pace feels smoother, breathing becomes more controlled, and the run begins to settle into a rhythm. This is when the body has successfully adjusted to the workload, with improved oxygen delivery and muscle efficiency. While this adjustment period is common, the speed at which the body adapts varies. Endurance-trained athletes can increase oxygen uptake more quickly than amateurs, thanks to years of training that improve their cardiovascular and muscular systems. Although the transition can't be completely avoided, strategies like a gradual warm-up—such as easy jogging or brisk walking—can help. These activities raise muscle temperature, increase blood flow, and activate the aerobic system before the main run. Starting slightly slower than your target pace can also ease the transition. Regular endurance training, including easy runs, tempo runs, and interval sessions, can further improve the body’s ability to adapt to exercise. There is also a psychological aspect to this experience. Experienced runners understand that the initial discomfort is normal and not a sign of a bad run. They trust the process and allow their bodies time to settle. As an ex-international runner, exercise physiologist, and coach, it’s important to remember that the sensations felt at the start of a run are rarely a sign of something wrong. More often, they indicate that the body is doing exactly what it's designed to do—adjusting to the demands of exercise before settling into a sustainable rhythm.