You should increase your rides by no more than 10% (I think thats the number) in any one week.

If you can ride 80 or 90 miles, then you will be able to do 100 even if it is your first time.

If you want to do 100 miles in a specific time, thats when you have to lots of long distance training rides, and work on your endurance.

Good luck and have fun. This is one type of cycling that doesn't appeal to me, and I am constantly impressed by the number of women here at TE that do such primo jobs of riding these distances on a regular basis.