Well that was interesting; first 25+ miles of my ride yesterday I was feeling fantastic; was struggling to keep it into zone 2 and not go harder. Was averaging about 170 watts and about 125BPM; then, inexplicably, pedaling became difficult to generate more than 100 watts. It may have been because I didn't eat enough; or maybe my legs can only do 170 watts consistently for an hour and a half before hitting fatigue. Nevertheless, I forced myself to push harder and maintain at least low end of zone 2. I also missed a turn and added 10 miles to my ride; which meant I got back pretty late (was getting dark). I recovered somewhat on the last leg of my ride, and my heart rate was higher per watts produced than it was during the first third of the ride (I've noticed this happen when I'm fatigued).
This morning my resting HR was about 45.5 (once it settled down it was very stable between 45 and 46). Last 30 seconds standing HR was about 58 BPM.