Participants will receive an MRI of the Fontan circulation. This will include imaging of the heart, lung and liver. A blood draw (approximately 10 ml) for assessment of serum biomarkers and circulating microRNAs of interest will be completed. The participants will also complete exercise testing and will then start a 3-6 month long cardiac rehabilitation program. Frequent phone follow up (conducted by the research staff) and regular download of data from heart rate monitors will ensure adherence.