Long slow distance (LSD) is a form of aerobic endurance training used in sports including running, rowing, skiing and cycling. Physiological adaptations to LSD training include improved cardiovascular function, improved thermoregulatory function, improved mitochondrial energy production, increased oxidative capacity of skeletal muscle, and increased utilization of fat for fuel. Ernst van Aaken, a German physician and coach, is generally recognized as the founder of the long slow distance method of endurance training.

Long slow distance training is a form of continuous training performed at a constant pace at low to moderate intensity over an extended distance or duration. The moderate training intensity of LSD is effective in improving endurance and maximum oxygen uptake in individuals who are undertrained or moderately trained. Although Long slow distance training is not effective when used in isolation by well-trained athletes, there is substantial evidence that Elite athletes spend 70% percent or more of their training time at LSD output levels, that Long Slow Distance effort levels are a necessary part of the training of world class athletes, and that LSD workouts are primary drivers of the lower resting heart rates seen in well conditioned athletes.