Doing an organized ride is vastly different from running a race. There are no spectators out cheering you on. There are, however, plenty of cars who will be pissed at you if you block the road. Oh, yeah, the roads aren’t closed. Water and nutrition are available, but don’t expect anyone to stand by the side of the road handing it out. Finishes are not anything “official” meaning, if you don’t report that you’re off the course, nobody will be out looking for you. If you do come in to say you’ve finished, they might smile and say congratulations. They may simply direct you to the food.
They are alike in that the fun-factor and/or challenge level is up to you and how you approach the event. Think it’s going to be impossibly hard, and it will be. Go into it with the mindset that you’re in for a beautiful day, you’ll have a better chance of seeing the beauty all around you.
And when your back is hurting with 40-50 miles of steep climbs ahead, a few works of encouragement, and a laugh when you admit that you’re thinking of going “metric” is all it takes to keep your chin up. This is especially true when the turn off for the metric century is 20+ miles behind you.