As others have said, most runners may 'technically' have bradycardia. This is not necessarily interesting as the number is a bit arbitrary, unless your heart rate is dropping significantly from your baseline of ~50-60 or you have symptoms of lightheadedness from the heart rate itself.
Basically you are almost not getting enough blood to perfuse the brain upon standing. It sounds like orthostatic hypotension but would probably need to rule out an arrhythmia particularly in your age and endurance athletes are at increased risk of afib, or something else. I would get an apple watch to monitor for afib and talk to your doc (they see this sort of stuff all the time). You can check for orthostatic hypotension by measuring your blood pressure laying flat, then sitting, then standing and really measure again after 2 minutes standing, obviously do this with someone around so you don't faceplant.