Hello I Am A Dedicated Endurance Athlete (Ironman) And Have

hello I am a dedicated endurance athlete (Ironman) and have a resting HR in the low 40's. I went in for a physical today and my doctor saw an "incomplete right bundle branch block" on my EKG. He said it was right on the edge, or just beginning? I am scheduling a follow-up with a cardiologist to be sure my heart is ok given I am training for a marathon. He's my new primary care doctor, and while I've never been told to be concerned about an EKG before, I honestly can't remember if this had shown up in the past and my doctor attributed it to my endurance training. Is this a common reading for endurance athletes?