Well, here's my 02....
Last Saturday I was in the parking lot waiting to go out with the distance builders in my club (30-50 miles at 15-16mph) and on somewhat on a whim decided to join a group heading out on a planned 87 mile ride to a remote bakery in North Texas and back. We had another group leaving from another location planning to go 100 at 15-16 mph. The farthest I had ever been previous was 52 at 15 mph. I ended up hanging on for dear life then getting dropped before making the bakery. Pace started at 17mph and got faster. I had a GPS based map and a GPS unit (first ride with it) so told the group to go ahead. I did get to the bakery without problem and had a great lunch with some extra carbs! I waited on the DB paced group to go back with. Ended up at 97 miles and ave of 15.3 mph. Would have gone an extra three but my GPS unit ran out of juice and I didn't know I was that close. I should have left my Cateye on my bike until I got used to the GPS but didn't and learned a lesson. I really believe that if you go 40-50 miles regularly and go at a comfortable pace it is pretty doable to go 100. I was pretty fatigued at the end but not too beat up to ride on Sunday.
The temp was in the 30s and I was dressed appropriately. I had two power bars for the trip and ran out of water right at the end of the ride. Of course we did have lunch at the bakery so it allowed for a good amount of recovery. Also was fortunate to catch a group coming back that was rolling at a comfortable pace for me. I was on my 06 Tarmac Pro. I would classify myself as an intermediate rider. I weigh 255 lbs and am trying to lose some weight and get in better general shape.
I can't say that had I specifically trained for the near century I might have been less fatigued but I didn't feel too bad at the end. Maybe I inadvertantly have been training for a century by riding long on weekends and adding a couple of short rides during the week. Of course nobody in Dallas has ridden much this year as the weather has been terrible.
Here is my GPS link:
http://trail.motionbased.com/trail/activity/2062697
Keep in mind that it was my first ride with the GPS unit and I did not have the autostop set so it counted non-rolling time and speed. Also I managed to pause it for about 5-6 miles. Live and learn I guess.