Max Heart Rate

That's really interesting.  I'll be curious to see what others say.

What jumped out at me from your Garmin data, is that you have a +10bpm jump in HR at ~20 min, but no corresponding increase in elevation, and your pace actually decreases and then becomes more erratic, especially compared to your earlier pace, again, without a corresponding elevation change.

Just out of curiosity, what's your BP like, if you don't mind sharing?

HR can vary so much for individuals.  You have a high max heart rate for your age, but the equation of 220-age is just a guideline.  You are at one end of the curve--according to the posted data you hit 206.

When you have these runs that are 190+ what is your PRE?  Does it feel the same as when you're doing a 160bpm run?  That probably seems like a silly question--but I'm trying to understand if you're specifically trying to push to your max running or if it just accidentally happens and you notice it when you're going over data.

Good questions.  My BP has always been quite low. I forget the numbers 115/60 or so.  That 206 day was "lets see how high I can get this" when I saw it riding at 190 and about .4m to go.  PRE was higher than my 180-190ish workouts but not a lot and I felt fresh and "fast" (for me)  like i really wanted to push past anything.  If that makes sense.  

That jump at 21min is a bit suspicious, granted but it would be a strange way for a data anomaly to show up.  Especially since it steadily went up during the 5 min stretch at the end. It was at my turn-around, so a quick stop and a quick first several steps to "catch up" may look like a non-change pace-wise as the 305 calculates and averages out that 10 second span.  But still a power/exertion spike that triggered the HR jump that settled down a little 2 min later along with my pace.  Slightly.

Just kind of interesting

I may "stress-test" my self this week and see what my top-end is now.  

OK, given that you have low blood pressure, I'm going to ask you a couple additional questions:

When you stopped to turn around, could you have bent over to tie your shoe or done something to put your head lower than your heart for a bit?

Do you ever notice if you have pooling in your extremities (edema), particularly in your legs, during long runs, long drives or long periods of standing?

No, on the stopping and head down vasovagal thing, and none of those edema type symptoms, and never diagnosed with anything like that.  Only comments I get at routine BP checks are positive and encouraging,  However, a couple of my first couple of open water swims back in '11 (my rookie year) resulted in what I later found out was SIPE.  The first one was moderate I'd say, pretty severe congestion and slightly pink "stuff" and felt like a heart patient the rest of the day (exhausted just walking up the stairs back home) fully "recovered" in 24 hrs, the second time I knew it was coming and backed off early enough and it wasn't a problem.   Never experiences it in any of my dozen or so Oly tris since.

Thanks for your interest, I will keep documenting my workouts.