We performed 6-yr follow-up testing on a world record-breaking master marathon runner. We determined that his performance declined since his record-breaking marathon in 2018 primarily due to a reduction in V̇o2max. His max heart rate (HR) changed minimally, but his peak O2 pulse decreased, suggesting that his maximal stroke volume and/or arteriovenous O2 difference decreased. These changes likely reflect primarily age-related effects in the absence of an overt pathological disease process.