I agree that a huge part of long rides is mental. For me, the mental game starts somewhere around 80% complete, regardless of ride length. It happens with metric centuries, with imperial centuries....distance doesn't matter, I just want to be DONE.

No centuries last year...maybe I'll do one this year. For me, metrics are the perfect distance- long enough to be a "long ride" and short enough to leave me with energy for the rest of the day.