In my experience (which is not that extensive at long distances), longer distances ridden slowly are hard in different ways than shorter distances ridden (relatively) quickly, but not necessarily harder.
The big problem I find is that there are some biomechanical problems that only appear above a certain threshold, for example, I only start getting hotfoot after about 115 km. So I need to train beyond these thresholds and figure out how to avoid these problems (for hotfoot: take a break at 100 km or ride with shoes looser). But I've also found that if I'm taking it really easy on a really long ride, a lot of the muscle soreness that I expect after a shorter ride just isn't there.