Well, I was a runner, mostly on a treadmill, up to the age of 67, but I was a duffer compared to you.. I was a bit heavier too at about 230 pounds, but 6' 6" helped carry the weight.
I think it good to get a check-up, when was your last full physical exam?
You don't want to see any 200+ BPM ever.
Is you pulse regular? Atrial Fibrillation is more common among us seniors, age. It is usually intermittent at first so the doctor may not see it, but it you get repeated runs of high HR you may need to wear a medical heart monitor for the doctor to examine.
I'm the SAME GUY - 20 years younger.
My profile pic is a Garmin Connect graph of me in multiple SVT's.
HR... well, the monitor could be off. If it's not strapped tight then it's funky and worthless. I've had those issues when I get drenched sweaty and the elastic doesn't grab my chest well enough.
If that's not the problem and you did hit 205... how long? Did you FEEL it??
I *always* felt it coming. As a runner you know... rhythm. Feet, breathing, arms, heart... everything works like a clock. Breath, stride, stride, swing....
However, my heart would get out of rhythm (literally). It would feel like it was "floating" in my chest... then BOOOM. POUNDING Heart. I'd stop, recover it and it would go to the same rate as if I were recovering for the same amount of time.
You get THAT?
Running in heat. It was hot this summer and I did have some runs where my HR did just go throught the roof very quickly. (speed and heat) It wasn't SVT, but the HR was rapid and quick to onset. (I did not wear a monitor). Based on my experience, that was "normal." I do not believe I have had an episode of SVT since my ablation in 2010.
So, I don't know know if I've answered anything for you. But you could get a Holter monitor and try to "tempt the devil." See if they can get a recording of the rhythm. That's the first step.
Good luck. I'm trying to BQ in November. That's a far shout from laying in the road!