I'll also add that one of the factors that exacerbates the issue is discomfort. Often this is saddle related, but it also can be caused by feet (hotfoot), hands, shoulders, neck or knees. Cramps also can creep in. And there is that nasty issue of groin and butt chafe let alone sore sitbones. Putting up with any of those things can make the time taken to complete the last 25 miles seem like an eternity.
People who want the quick fix in their training dismiss Long Steady Distance as a legitimate early-season training method, but it has other functions besides improving fitness... it can identify the sorts of fit issues that are going to cause pain, and it can allow the body to get used to being outside its cushy comfort zone.
Three or four-hour rides each week will help whereas 90-minute HIIT sessions won't.
Of course, HIIT comes into its own when there is a solid base and a desire to build speed. But even then, there needs to be experimentation on nutrition and hydration at higher intensities over long periods. And so we come back to the issues of negative thoughts being related to food and fluids.