Endurance performance declines with advancing age. Of the three main physiological factors that determine endurance running performance (maximal oxygen consumption [V̇O 2 max], lactate threshold, and running economy [RE]), V̇O 2 max appears to be most affected by age. While endurance performance declines with age, recently, endurance performance has rapidly improved in master athletes as the number of master athletes competing in endurance events has increased. Master athletes represent an intriguing model to study healthy aging. In this case study, we reassessed the physiological profile of a 76-year-old distance runner who broke the marathon world record for men over 70 years of age in 2018. This runner was tested a few months before breaking the world record and retested in 2024. Between 2018 and 2024, his marathon running velocity decreased significantly. Therefore, the purpose of this case study was to determine the physiological changes that explain his performance decline. RE remained similar to 2018, and while there was not a clear breakpoint in blood lactate, he still likely runs marathons at a high percentage (~90%) of his V̇O 2 max. However, V̇O 2 max declined by 15.1%. HRmax declined by 3.2% and maximal O 2 pulse declined by 12.4%, suggesting that maximal stroke volume and/or arteriovenous O 2 difference decreased. Altogether, although this marathoner continues to compete at an elite level, his performance has declined since his record-breaking marathon due to a reduction in V̇O 2 max. This is likely caused by reductions in maximal stroke volume and/or arteriovenous O 2 difference. We speculate that these changes reflect primarily age-related processes.