I think this is the key points to the artical and people shouldn't get freaked out but understand why people say "Check with your Doc first."
Activities such as marathons, iron man distance triathlons, and very long distance bicycle races may cause structural changes to the heart and large arteries, leading to lasting injury.
Lead author Dr James O’Keefe, of Saint Luke’s Hospital of Kansas City, said exercise was generally beneficial for health but could tip into becoming harmful when taken to excessive lengths.
Good read Steve. ^5