I've been told by several experienced cyclists that if you can ride 60-70 miles two days in a row you can handle a century. I would think the weekends leading up to this would need to be include challenging rides (hills, long climbs, etc) to condition your body. You might try increasing your weekend milage by 10% each week. So say you ride 25 miles on Saturday and again on Sunday, try the next weekend to bump that up to 28 miles each day and continue to do this until you make it to your goal of 60 - 70 miles both days.
Good luck and keep us posted on your progress.