Well, I work out four times a week and then play hockey on Saturday which gives me a seriously good cardio workout.
And when on the bike and treadmill I monitor my heart rate which rarely goes above 140 even when peddling hard or running at 11.5 km/h.
I'm assuming that means my heart is ok. Mind you these stories about fitness nutters dropping dead of a heart attack are always a bit worrying. There was one recently, chap was the England hockey coach and a fitness addict and he had a heart attack out of nowhere.
At 46 or something.