AbstractBackgroundWalking slows with aging often leading to mobility disability. Mitochondrial energetics has been found to be associated with gait speed over short distances. Additionally, walking is a complex activity but few clinical factors that may be associated with walk time have been studied.MethodsWe examined 879 participants ≥70 years and measured the time to walk 400 m. We tested the hypothesis that decreased mitochondrial energetics by respirometry in muscle biopsies and magnetic resonance spectroscopy in the thigh and is associated with longer time to walk 400 m. We also used cardiopulmonary exercise testing to assess the energetic costs of walking: maximum oxygen consumption (VO₂peak) and energy cost–capacity (the ratio of VO₂, at a slow speed to VO₂peak). In addition, we tested the hypothesis that selected clinical factors would also be associated with 400‐m walk time.ResultsLower Max OXPHOS was associated with longer walk time, and the association was explained by the energetic costs of walking, leg power, and weight. Additionally, a multivariate model revealed that longer walk time was also significantly associated with lower VO₂peak, greater cost–capacity ratio, weaker leg power, heavier weight, hip and knee stiffness, peripheral neuropathy, greater perceived exertion while walking slowly, greater physical fatigability, less moderate‐to‐vigorous exercise, less sedentary time, and anemia. Significant associations between age, sex, muscle mass, and peripheral artery disease with 400‐m walk time were explained by other clinical and physiologic factors.ConclusionsLower mitochondrial energetics is associated with needing more time to walk 400 m. This supports the value of developing interventions to improve mitochondrial energetics. Additionally, doing more moderate‐to‐vigorous exercise, increasing leg power, reducing weight, treating hip and knee stiffness, and screening for and treating anemia may reduce the time required to walk 400 m and reduce the risk of mobility disability.