Excellent (and yes regarding Maffetone), and one final thought — as just back from a run.

Sam Marcora has being doing a considerable amount of work on his psychobiological model. He suggests that mental fatigue impacts perception of effort, and that causes us to discontinue/reduce effort. I am a very slow ultra-marathoner but I think i benefit from running twice a day. The second run is on tired legs, but perhaps more importantly I am also mentally fatigued after a day at work. It may provide some sort of resistance/resilience. I know Marcora has tested subject in the lab by mentally fatiguing them, and it has negatively impacted endurance performance. He was even working on an app to use prior to running. For all of us it could be mental fatigue we need to train, to prevent a withdrawal of effort. I realise much of this is theoretical, but may offer some ideas.

Cheers, Jeremy