At some point during Day 2, 3, or 4 of every single damn 600+ km event there is a low spot if I’m by myself (which is very likely). Right around when the temperature (high or low) starts getting unbearable and hills all feel super steep to my smashed legs and sleep deprived brain.
You are correct though. That is an untrainable hurdle except through experience, mental fortitude, and a significant other who knows how to text Rule #5 to you.
Regarding training plans, the sustained power build workouts are awesome for long distance riding. Cruising all day at 50-60% of 300 watts is way faster than cruising all day at 50-60% of 240 watts. And that means an extra hour or two of blessed sleep per night.
And because of our rando riding, our short power SUCKS so it’s worth doing that solely so that when the roadies catch up during a hill climb on their carbon steeds with no extra weight, you can make 'em earn that “I crushed that dude on the 30 lb steel bike with both a handle bar bag and huge trunk bag” smirk they share at the coffee shop after their epic 32 mile ride. Or at least prevent them from having an easy conversation as they pass you…