At its core, endurance running depends on your aerobic metabolism. The aerobic system (the body’s ability to produce energy with the use of oxygen) is the engine behind long-term, sustainable performance. LSD runs specifically target and strengthen this system by keeping the runner in a low-to-moderate intensity zone for an extended period of time, often 60 minutes or more and sometimes for several hours depending on the person’s goal. Because the intensity remains low, the body can operate almost entirely aerobically, allowing for development of the aerobic base without incurring excessive fatigue or requiring long recovery periods between runs.
One of the most important physiological benefits of LSD running is the increased ability of muscles to use oxygen efficiently. During long, sustained efforts, the body adapts by increasing the number and density of mitochondria (the cell’s energy-producing structures). More mitochondria allow muscles to produce more energy over long periods without fatiguing. LSD runs also enhance capillary density, meaning more tiny blood vessels are available to deliver oxygen to working muscles and clear metabolic byproducts. These adaptations take time, consistency, and repeated low-intensity stimulus, which is exactly what the long slow distance run provides.
In addition to mitochondrial and capillary adaptations, LSD runs strengthen the heart itself. The extended duration of these runs encourages an increase in stroke volume (the amount of blood the heart can pump with each beat). A stronger heart pumps more efficiently, reducing the need for a high heart rate during all types of exercise, even during high-intensity sessions. This is why a well-developed aerobic base often allows runners to execute speed work with more control and better form. Simply put, the stronger and more efficient the cardiovascular system is, the more latitude a runner has to push harder during demanding sessions without tipping into excessive fatigue.
Another crucial benefit is the body’s improved ability to utilise fat as a fuel source. At lower intensities, the body relies more heavily on fat metabolism, which is nearly unlimited compared to carbohydrate stores. By regularly running at LSD pace, athletes train their bodies to become ‘fat-adapted’, meaning they can spare valuable glycogen for later stages of long races or for higher-intensity efforts. This is especially important for marathoners and ultrarunners, who must avoid depleting glycogen too early in the race. The long slow run essentially teaches the body to be more energy efficient across all paces.
Beyond physiological improvements, LSD runs also play a major role in building musculoskeletal durability. Running for extended periods strengthens connective tissues such as tendons, ligaments, and the fascia. These tissues adapt more slowly than muscles, which means gradual conditioning is essential. Consistent long slow distance runs provide the controlled stress necessary for tissues to become stronger and more resilient. This reduces injury risk and increases a runner’s ability to handle higher training volumes. Without this structural conditioning, harder workouts and faster runs would place too much strain on the body, eventually leading to injury.
Just as importantly, they build mental toughness and familiarity with time on feet - skills that are often overlooked but essential for endurance performance. The psychological experience of being out on a run for over 90 minutes teaches patience, pacing discipline, and comfort with sustained effort. These mental traits translate directly to race day. Runners learn to stay relaxed, manage slight discomfort, and remain focused during long events. They also develop strategies for nutrition, hydration, and maintaining form during the later stages of fatigue. No other workout simulates these race-specific demands as realistically as the long slow run.
Although speed workouts like intervals, tempo runs, and hill repeats are vital for improving race-specific performance metrics such as lactate threshold and running economy, these sessions depend on a strong aerobic base to be effective. Without the foundation provided by LSD running, high-intensity training becomes more taxing, less productive, and more likely to cause injury. The long slow distances essentially support the entire training hierarchy.
Another often overlooked aspect of LSD runs is the opportunity they provide for recovery and consistency. Because intensity remains low, these runs help maintain weekly mileage without overloading the neuromuscular system. They also promote blood flow and active recovery, helping the body heal from tougher sessions earlier in the week. Many runners find that LSD runs allow them to accumulate significant training volume, whilst still keeping the overall stress level manageable.
There is also a pacing benefit. LSD runs teach runners how to regulate their effort intuitively. Many athletes, especially newer runners, struggle with going out too fast in races. Spending long periods at a controlled pace helps develop a sense of ‘slow,’ ‘hard, and ‘too fast.’ This pacing awareness is a competitive skill, one that can save enormous amounts of energy during a race itself.
It’s worth noting that ‘slow’ is relative. The proper pace for an LSD run is typically conversational, roughly 70% of maximum heart rate or an effort where breathing remains steady and speech is easy. Many runners mistakenly push their long runs too hard, turning what should be an aerobic-building session into a taxing workout that delays recovery. True LSD pace should feel gentle, sustainable, and somewhat relaxed. Paradoxically, the discipline of running slowly often requires more restraint than pushing hard.
Ultimately, the long slow distance run is indispensable not because it is glamorous or exciting, but because it cultivates the core qualities of an endurance athlete. It’s the quiet, patient work that makes breakthrough performances possible. Whether a runner is aiming to complete their first ultra or shave minutes off a marathon personal best, incorporating regular LSD runs into their training plan lays the foundation for success.
1,230 Marathons, 270 Ultras, 18 MDS, 10 GWR, Infinite LSD...
