I don’t ride with power outdoors. Hence cannot use the heart / power decoupling as a sign to bring a LSD ride to a close.
I do ride with heart rate outdoors. Something I noticed after I’d had a cold in October was that my early rides (after the cold passed) my RPE was divorced from my HR. What I mean by this is that I have a pretty good sense of my RPE and how that relates to my HR. They are anchored pretty well together. But after my cold my RPE was significantly higher than my HR and it’s rate if increase would indicate.
I’m therefore wondering if RPE vs. Heart rate can be a good measure for working out when to bring a LSD ride to a close. You’d calibrate the RPE vs.HR response in first hour. Then just keep an eye on it and when RPE seems much higher vs. HR compared to first hour. It’s time to bring it to a close.
My other thought is leg fatigue but since I ride brevets the legs can often just keep going and going.