I don't think the fatigue you described is limited to those who tour for weeks or months at a time.

My longest tour was about a week long and only took me two states away. The anticipation and excitement before the trip took a lot out of me before I even pushed the first pedal. Then, the reality of many hours of boring riding - coupled with the fact that none of the people I met along the way were as impressed by my undertaking as I was - kind of added more weight. I have great memories of that trip now, but right after I returned from it, I remember feeling like that would be my last "tour." In fact, I don't think I even got on the bike for several weeks afterward.

So, as you said, I think the "fatigue" part is largely psychological. I have always had - and still have - virtually limitless energy for any ride. It's only my head that gets tired looking at the pavement passing by for so many miles at a time.