I've done the 112 mile version a couple of times and it's the most exquisite torture I've done on a bike in a long time. Just gorgeous scenery but lots of climbing. I did it on a road bike riding 32's. I ride a triple with a 30/28 low. I managed to ride all but about 100' of a dirt road "wall" that would be easier on a Mtb because of balance issues.
People did the ride on road bikes w/ 25's but get ready for more potential flats and more bike handling issues. It's not a ride, IMO, to do for time/speed. Unless you're in Cat 1/ pro shape it's better to just settle in, do the ride and enjoy the scenery.
For training I'd do as much long distance with climbing as possible.
So sorry I can't do it this year due to work. This is a great ride and you won't regret the effort. Enjoy it and let us know how it goes.