Heart Failure Congenital

The goal of this clinical trial is to compare the impact of cardiac rehabilitation on Fontan failure patients' exertional tolerance, frailty, and quality of life. 1. Among patients with Fontan failure, will cardiac rehabilitation increase average daily steps compared to usual care? 2. Among patients with Fontan failure, will cardiac rehabilitation improve exertional tolerance (as measured by cardiopulmonary exercise testing), frailty, and self-reported quality of life metrics compared to usual care?