Participants will undergo an MRI of the Fontan circulation. This will include imaging of the heart, lung and liver. This will include specific imaging for tissue characterization and assessment of myocardial fibrosis, liver fibrosis and disproportionate pulmonary blood flow. The investigators will then draw blood (approximately 10 ml) for assessment of serum biomarkers and circulating microRNAs of interest. The participants will undergo exercise testing and will then start a 3-6 month long cardiac rehabilitation program. After the 3-6 month study period the participants will return back for a follow up and repeat all the testing completed at enrollement.