What is your longest ride? How many days a week do you ride and how long are they?
Personally, I'd like to have a 25-miler in beforehand--preferably 30 if I wanted to finish comfortably.
The rule of thumb is add 10% to your longest ride each week. However, my BIL once did a century with me--his longest ride was 40 miles but he rode 15-20 miles almost DAILY. I don't recommend that, though.
"Well-behaved women seldom make history." --Laurel Thatcher Ulrich
'09 Trek WSD 2.1 with a Brooks B-68 saddle
'11 Trek WSD Madone 5.2 with Brooks B-17