If you want a training plan, British Cycling should have something on their site, or Prudential for the Ride London
The Ride London plans will be overkill. RL has the complication/pressure of a minimum required speed to beat the broom wagon which isn't a factor on century day trips. I suspect most Brutish Cycling plans will be aimed at getting to the finish by some time limit to get a medal or other toys before the finish packs up and goes home.
Don't worry about speed. Who cares as long as you can get home? Use all the tricks like freewheeling, pulse and glide and enjoying the views with "what's over that next ridge?" pondering. Even if you have to bail out, there's always another day, which is an option most sportive riders won't take.
As others (including
@vickster) say, build up the distances. If 30 miles is your current normal, then I'd suggest doing a 50miles, then two or three 100km with a lunch stop (which is basically only 2x 30 miles on the same day), then at least one 125km with a stop before doing 2x 50miles on the same day with lunch between them... hey presto you've done 100.