question: Is a resting heart rate of 43 beats per minute too low of a heart rate? I am 19 years old, 3rd degree black belt martial artist, run cross country, and participate in rigorous Marine physical training. However, I do not run faster than a 20 minute 3 mile. 20 minutes in 3 miles is somewhat average at my age I believe and I have never trained for or ran a marathon. Very respectfully, Patient C