Hello,
Thank you for your query at doctorspring.com.
A low heart rate is common in people who are accustomed to physical exertion.
Since you run around 15 to 18 miles every week you are physicaly well trained. You do not have any symptoms even if your heart rate is low. Also your heart rate increases normally with exercise. Search asymptomatic bradycardia, it needs no further medical investigation.
Hope this helps, would be glad to answer any follow-ups, clarifications.
Regards
Dr Vivek Mahajan
DM Cardiology