did my first 'century' (unsupported) ride without much prep. a buddy and I just decided it would be fun to ride a hundred miles.
my longest previous ride was 47 miles. didn't take any food, had one water bottle. wore cut-off Levis and Adidas running shoes with toe clips.
got lost, ended up riding 108 mi.
was tired as crap at the end, but really wasn't a big deal. just went home and ate everything in the house and fell asleep for half a day.
you should be able to do a supported ride without much grief.