So today I did a 6 hour ride in the lower two power zones. But my HR was almost near the threshold zone! Temperature wasn’t high, pleasant even. Hydration was ok about 750 ml per hour. My max HR is 200 ish and I did 153 AVG for 6 hours. I had about a week off and did two weeks with 63 hours of riding for vacation with about 1000 TSS per week. My normal TSS per week is about 600 with structure and free rides.
Does anyone have a clue what’s coming on?
Zone 1 and Zone 2 rides, especially long ones like what you did, can be harder than you think!
HR is so hard to use as a basis for judgement in this scenario. It is just about all anecdotal - so jumping in (with my anecdote) when I’m rested my HR reacts much more quickly than when I’m tired. If you were on holiday, granted having done 1000 TSS over the week, you may well feel more rested. As I say my two 'pennorth and mine alone.
My max HR is 190. Two centuries a weekend ago. Average HR was typical to what I usually see on zone 2 rides.
Saturday, 5:40, .64 IF, 147 average HR
Sunday, 6:00, .59 IF, 147 average HR
Can you share a link to the ride?
Was this a steady progression of HR from a lower level, or high from the start?
6 hour z2. About 75% of HRmax. Sounds about right to me.
You say it was way high…what is your normal HR reading over a 6 hr ride?
And what temperature was it. Hotter than normal?
My endurance pace at the moment is near sweet spot HR because of the heat. 5 to 10 degree hotter than I’m used to riding in.
On the weekends I do 2x 5hr Z2 rides: 68% HRmax @60% FTP. I keep it on the low side on purpose, a ‘freestyle’ Z2 ride would probably look more like your rides @~77% HRmax. 
