@rice It's one thing to have an exercise heart rate, but it's quite another to have an exercise heart rate while fibrillating. I agree with rice...the rule-of-thumb is max sustained heart rate plus one's age should not exceed 220. You're way over that.
I, too, am a life-long cyclist, runner (many, many races), and snowshoer. I went into AF at the tail end of a 10 km maintenance run the summer I turned 65. They did every conceivable test, all showing no ischemia or blockages. Finally my cardiologist ordered a polysomnography which revealed that I have 'severe obstructive sleep apnea'. What this meant is that 'for God know how long' I had been subjecting my heart to nocturnal marathon-like stress because it was trying desperately to oxygenate my organs against the 30+ obstructive events I was having...EACH HOUR....night after night!
This may or may not turn out to be your problem, and so far your other numbers indicate you're not in a lot of distress. That's good. And if you're asymptomatic, you're getting off very lightly indeed. For the rest of We Lesser Mortals, we are not only highly symptomatic, which is awful (believe me), but we deteriorate in a linear progression with the advancement of the electrical disordering of the heart's substrate. And advance it does...and will...in all cases. Some very slowly, some precipitously...fast! I went slowly at first, well controlled for three years with only a weak dose of metoprolol and apixaban, but then the wheels began to fall off. You are not there, clearly, and again, it's great news. I recommend staying on top of the typical changes that happen for AF patients whose hearts beat chaotically unbeknownst to their owners: mitral valve prolapse as a possibility and eventual heart failure...again...only a possibility, neither one a certainty. But they do happen to many AF sufferers, so please get regular checkups to make sure you're not getting atrial enlargement. Also, might be a great idea to get in the back pocket of a good electrophysiologist soon so that you can be evaluated for a catheter ablation or maybe only a Watchman device (you can google it and read up on what it is and why it can obviate having to take an anti-coagulant with its associated risks and expense).