Just to be sure - you've cleared your bradycardia as healthy with a doctor, right? If you're not exceptionally fit as you say, that could also be a sign of something being wrong.
I spent 36hrs in hospital after an asthma attack last year (3 days in a super dusty environment), it was noticed then but not to the full extent - I think after normalising the asthma symptoms with a bunch of salbutomol nebulizers (iirc) I was ~55. I'll ask when I next go to the doctors :)
Before I was diagnosed with asthma, I just knew that at some places (my mother in law's was common), I'd have poor breathing after a while. Just learned to relax through it, and the symptoms would lessen after I left.
I go to get diagnosed, and they give you a nebulizer with something that triggers symptoms if you have it. I didn't visibly react, so the tech did it again, then measured the difference in my breathing. Whoops, the lack of visible response meant that I should have stopped after one dose.
So, I couldn't leave until after enough doses of an inhaler that it went back to normal. About 7 hits, I looked like I was sunburned for a while.
This. You might want to get checked for arrythmia if you haven't. My dad registered as having bradycardia using the home devices. What was actually happening is that he had a more normal heart rate at like 70 to 80 but that the beats were irregular and some weren't strong enough to be detected by basic heart rate measurement devices. It was only an EKG that was able to accurately detect his real heart rate.