I don't have a written plan, but here's some advice:
Make one ride a week a "long" ride. Add 10% per week or 5 miles. (If the longest ride you have done this summer is 20, do a 25-miler next week). Add in a rest week on occasion, if you need it (weather or "life" will usually do it for you). Something like this:
25
30
rest
36
43
rest
51
rest
Metric
Work up to 50 miles or so, if you can. (My BIL did a century with a long-ride of 45, but I don't recommend it).
Work on hydration and what food you can tolerate and fuel you up. I don't rely on what I'm given at organized rides.
Find a few local organized rides if you can. In St. Louis, Trailnet does organized fun rides as long as 44 miles. Good way to find new routes and practice your "road" skills.
"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