The Marathon Study was funded by the British Heart Foundation (FS/15/27/31465), Cardiac Risk in the Young, and the Barts Cardiovascular Biomedical Research Centre. The study received support from COSMED through the provision of cardiopulmonary exercise testing equipment and technical support. Dr. Bhuva is supported by a doctoral research fellowship from the British Heart Foundation (FS/16/46/32187). Dr. D’Silva was funded by a clinical research training fellowship from the British Heart Foundation (FS/15/27/31465). Drs. Moon and Manisty are directly and indirectly supported by the University College London Hospitals, NIHR Biomedical Research Centre, and Biomedical Research Unit at Barts Hospital. Dr. Hughes has received support from the British Heart Foundation (PG/13/6/29934) and the National Institute for Health Research University College London Hospitals Biomedical Research Centre; and has worked in a unit that receives support from the UK Medical Research Council (Programme Code MC_UU_12019/1). All other authors have reported that they have no relationships relevant to the contents of this paper to disclose.