I'm reasonably fit - 250 miles a week on a bike, 200 miles hikes in 10 days or so a couple of times a year, lots of lesser stuff all year long ....
Recently a couple of weird "episodes" got me into a cardiologist for a work-up. (Blood, Stress, and Echo) He said "You are fit, we see nothing wrong, we see nothing suggestive, we could do more, but usually all we turn up is a bunch of False positives."
And although not very comforting, he then said "This is why you read about someone running a marathon and then having a heart attack the next week. It's an imprecise science. Go ride, go hike, and enjoy yourself."
So I am.