I did what Xavier said
...a 75 mile ride two weeks prior to the event. Before that I did 35 one day out of the weekend, next weekend a 45, next a 57 and then a 64 a week before the 75 miler. I then did a 44 on the sat. and a 30 on sun. a week before the 100. I also maintained my normal 20-30 mile rides 2x during the week except for the week before the event (just did a casual spin one day). I had only been back on the road bike for about 4-5 months after a several year hiatus. I worked with a group during the ride from Irvine, CA to San Diego and did it in 5 hours 10 minutes. You can do less training and just go slower, no biggie. Good luck.