Interesting
http://www.cyclist.co.uk/in-depth/1313/will-riding-use-up-my-heart-beats
Besides the worry about cumulative fatigue, the article mentions another downside: the significantly increased risk of arrhythmia. The incidence of atrial fibrillation is about fivefold greater in endurance athletes (esp. older males), and it tends to become more frequent and more persistent over the years. Even permanent AF can be compatible with almost undiminished quality of life and exercise capacity, but it comes with a risk of stroke (from blood clots formed in the heart).
See for instance Matthias Wilhelm, "Atrial fibrillation in endurance athletes", European Journal of Preventive Cardiology 2014, Vol. 21(8) 1040â€"1048 , which does say reassuringly “Although the aging endurance athlete has an increased risk of developing atrial arrhythmias, the benefits of regular exercise and an athletic lifestyle far outweighs the negative consequencesâ€.
http://www.stopafib.org/