<p>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.</p>