Add intervals of intensity for a stronger heart.  When it comes to exercise, interval training offers the best of both worlds.
D015444:Exercise / Q000502:physiology*; D006331:Heart Diseases / Q000534:rehabilitation*; D006333:Heart Failure / Q000534:rehabilitation; D006801:Humans; D010809:Physical Fitness / Q000502:physiology*; D013177:Sports / Q000502:physiology; D013997:Time Factors