How's your recovery after your normal ride? Are you sucking wind and can't move for hours, or do you feel strong & refreshed quickly? If you can recover fairly quickly, then (in addition to the great advice above) break down the 50 into smaller rides w/ a break in between. Use those rest stops to recover then go for the next ride, break, ride, done!

Mentally this helped me to get thru my first Long rides by thinking of them as a sequence of short rides - go & have fun!!