This study evaluated whether sustained participation in multi-marathon running is associated with elevated aerobic capacity, more gradual age-associated differences in VO2max, and reduced estimated all-cause mortality risk. A global cross-sectional online survey collected self-reported data from 340 multi-marathoners (mean age = 52.2 years; 54.1% men) representing 24 countries, with an average of 121 lifetime marathon completions. VO2max was estimated using wearable fitness device data and analysed by sex and age group. Estimates were compared against normative VO2max trajectories from the FRIEND (Fitness Registry and the Importance of Exercise: A National Database) cohort. Mortality risk was modelled using published meta-analytic data indicating a 3.7% reduction in all-cause mortality per 1 mL/kg/min increase in VO2max. Participants exhibited substantially higher VO2max values than population norms across all age groups, with multiple age bands exceeding the 95th percentile threshold. Compared to normative data, multi-marathoners showed a more gradual age-associated pattern in VO2max, particularly among older adults. Although men displayed higher absolute VO2max, older women showed greater relative advantage. Estimated mortality risk was notably lower across the cohort. These findings suggest multi-marathoners maintain superior aerobic fitness into later life, supporting a model of healthy ageing; however, potential cohort effects warrant longitudinal research to clarify causality.