Regular exercise improves cardio vascular functioning. Your heart will work more efficiently and the stress of fast running and other extra exertion will cause less adverse stress to the heart. Your normal resting rate will most likely be lower, that is better for you, than if you were taking no exercise. It is those who really cannot, or will not exercise who have hearts at risk.
Reglar exercise is, of course, better than intermitten exercise. A normal resting heart rate is between 60 and 100 beats per minute and you would want this to be at the lower, not the upper end of the range.