Endurance in marathon running is measured primarily by an athlete's ability to sustain physical exertion over the entire race distance, typically 26.2 miles. This is evaluated through pace, heart rate monitoring, and lactate threshold tests, which help determine how well runners can maintain speed without fatigue. Training regimens aimed at increasing aerobic capacity and stamina also play a vital role. Assessments through races, timed trials, and physiological tests offer insights into an athlete's endurance performance.