I wouldn’t put too much stock in generic heart rate zones, there is just too much variability. Most heart rate zone calculators just take your max heart rate into account, but not resting heart rate.
Mine is 45–50 bpm these days. During workouts my heart rate stays low. I have to push really, really hard to get it beyond 165 bpm. Even long efforts at FTP just get me up to 160–165 bpm.
More important than the number, I’d look at how quickly your heart rate decreases in recovery valleys. How many seconds or minutes does it take to go from heart rate at FTP to Z2 heart rate? If that happens quickly, usually that’s an indication of good fitness.
That workout was just about perfect for you if aerobic endurance was your goal. Looks like max HR from your last ramp test was around 165. You were in that 70% to 75% of max heart rate zone during a lot of the Baxter workout. Bob’s your uncle. Good job.
You can adjust your HR zones manually on intervals.icu if you want. I have mine set to match Ronnestadt’s HIT/MIT/LIT HR ranges, for example.
Honestly I was using an estimate of around 170. At this point, I haven’t done much digging into setting a more scientifically determined HR max… I know on ramp tests I tend to bail around 165-ish.
I had one outdoor ride over the summer where I averaged around 160… I was looking at it the other day… I was sort of surprised… but that was one of my best efforts last year… Trying to hang onto a group of guys faster than me and not get dropped.