Reddit reviews The Doctor's Heart Cure: Beyond the Modern Myths of Diet and Exercise: The Clinically-Proven Plan of Breakthrough Health Secr

Joking aside, long endurance exercises aren't that good for your circulatory system. HIIT (or Tabata cardio) is much more effective to strengthen your heart. I think Al Sears was the first guy to popularize this in his The Doctor's Heart Cure (or at least it was the first I heard of it), and if I'm not mistaken it's now the general consensus.

(And Mr. Fixx stopped smoking and lost weight 17 years before he died of a heart attack. Whether his heart was congenitally enlarged is open to question. His diet has been suggested as the big problem--he had the idea that since he exercised so much he could eat anything, and he was an inveterate carbohydrate hog--donuts, pies, cakes, etc.)