A multicenter analysis of 561 individuals with Fontan circulation found that surgical conduit size, ventricular morphology, and Fontan subtype had no significant association with exercise capacity measured as percent predicted peak oxygen consumption. Sex emerged as the strongest predictor, with females showing 12% higher exercise capacity than males, while age decreased capacity by 0.8% annually. This challenges conventional assumptions that surgical technique optimization drives long-term exercise outcomes in single ventricle patients. The findings suggest that progressive pathophysiological changes inherent to Fontan circulation itself, rather than specific anatomical repairs, primarily determine exercise limitations over time. This has profound implications for surgical decision-making and patient counseling, indicating that optimizing conduit diameter or surgical approach may have less impact on quality of life than previously believed. The research shifts focus from surgical perfectionism toward understanding and potentially mitigating the systemic circulation changes that develop regardless of initial repair quality. However, as this preprint awaits peer review, these paradigm-shifting conclusions require validation before changing clinical practice. The study represents confirmatory evidence that Fontan physiology's inherent limitations may overshadow surgical variables in determining long-term functional capacity.