you'll definitely need to get used to more miles on the bike. it sounds like yours isn't part of a race or anything, so you should be able to take a rest stop now and then. that will help.
i did my first century last year and it went great. but, it was part of RAGBRAI, so i had already been riding 50-70 miles on the three days prior. plus there were plenty of stops for food and drink along the way. i would say you should at least train up to 75 miles. after that, pushing another 25 or so isn't too bad. it would also help if you could do a couple back to back long distance rides (on a sat-sun, 60 each, for example).
as far as a routine, just get more miles on consistently and build up to 75 or so over the next month and a half.
good luck! it is a great feeling.