Still, not too bad, bmc. You'll have to talk to augi, or locowelder, or some of the other long distance riders, but I believe that these sorts of rides are the kind of thing you have to work up to, to get your body into shape.
What kind of bike/motor are you riding? If it's a cruiser, I guess I'm not that surprised... they can pound the bejeebers out of you after a few hours... A suspension seatpost will help, but a good set of shock front suspension forks helps at least as much as the seatpost, and (I'm told) a full suspension bike helps as much again.
The difference between my old cruiser and the trek navigator 'comfort' bike I'm on now is night and day.