I have not encountered any formal research results to support what I am about to say, but I have read opinions by medical personnel who treat people for heart rhythm problems. Take this for what it's worth: cyclists and runners, especially, and those who are either, AND who are tall, have a significantly higher probability of developing heart rhythm problems later in life. We're not talking a huge jump in the statistic, but enough that those working in the field will tell you that many who repeatedly stress their hearts with higher intensity training early in life show up more than those who don't push themselves quite so hard.

A healthy and well-trained heart will rest below 45 BPM, and that's into bradycardia territory officially. However, a knowledgeable physician will instantly dismiss this low rate once she learns of your physical training. And, I am a life-long runner, often competitive. My resting HR was about 38 for much of my adult life. I now have AF, and recently had a touchup ablation of the pulmonary vein ostia. Take it from me, you want to avoid having any heart arrhythmias if you can possibly avoid it. They tend to progress once you have them. So, continue to be fit, but be aware that you might develop fibroids inside your atria from scarring if you persist in very high intensity training for long. The more fibroids, the likelier you'll have heart problems.