Thank you so much for posting this. I literally just went through this same thing a few days ago, to the point where I am currently lined up for a 2D cardiac echo in July, and just returned the EKG halter they had me wear for a week. I don't know the results yet, but I'll let you know if they find anything interesting.

I'm 45 years old, and my resting heart rate is low 50's. However, one trek up my stairs to the second floor, and my HR is at 150 within just a few seconds. I can peg my heart rate at 200 easily on tough climbs, but for most of my workouts, it hovers at 175 to 185. Like you, I have one cup of coffee per day in the morning. Crazy similar.

My 20 minute LTHR is 176 (Trainer Road on Tacx Neo). It was the notion that my Max HR should be 175 (220 - age) that made me wonder if there isn't some sort of medical issue. As I'm already aware, the "max heart rate" is not a hard line, but the warning is that exceeding that number increases the chance of a cardiac event. So for me, I felt like I was playing Russian Roulette every time I fired up Trainer Road!

If my doctors do discover anything, I'll be sure to let you know, but again -- thanks for posting. Glad I am not completely alone.