Rowan offers some great advice above. I agree with all of it except this part:
Originally Posted by
Rowan
And yeah, the reason why other people do so well is because they indeed are prepared to put in two to three times the distance in training on the bike. Up your training a bit, at first to develop your routines and stamina. You can incorporate pace work later to help improve your event speed.
I personally don't think there's any reason to increase the amount of training you're doing. If you can finish 200k in 8 hours, then you have all the fitness you need to finish a 1200k.
Successful randonneuring requires some physical fitness (like I said, you have that) and some learned skills. The learned skills cover a broad range from the basics like knowing how to ride a bike, fix a flat and read a cue sheet, to more elusive things like knowing how to pace yourself, and how and when to eat and drink. Some people pick up those learned skills quickly and for others it takes some practice. I think if you slow down and focus on your body's fuel and hydration needs, you'll quickly pick up the skills you need to finish an SR series.