Ironically, I think this is partly true. Someone feel free to correct me if I’m wrong, but I believe the human heart generally has a roughly constant number of beats between people of a variety of levels of fitness. One difference is that people with higher cardiovascular fitness tend to have lower pulse rates when they’re not exercising (which is most of the time), so it works out that it takes longer to reach the average number of total heartbeats than for someone who doesn’t exercise.