It seems to be miles 40 to 60 for me, at least mentally. I feel even more tired than the last 20. In my mind, mile 60 puts me well into the last half. One of my annual centuries (The Rodman Ride for Kids in Foxboro, MA) has rest areas well-spaced at every 20 miles, and the one at 40 miles is always the most festive, so maybe there is a let-down when departing that one.