Originally Posted by
stevage
...(The 10% increment advice was bizarre and impractical. Who has time for that? 
)
A lot of folks have used that as a training guideline over the years. It's usually stated as increasing one's weekly mileage by ten percent each week. The main point is to keep pressing forward, but don't do huge jumps in training early in the game. It's a good recipe for avoiding injury, partially because muscles get stronger faster than tendons and partially because as you practice you will hopefully gain improved body knowledge and make small adjustments in your technique that improve efficiency and reduce strain on the joints.
That said, a one-off leap in distance for an event is different than bumping up one's training. First of all, you will likely back off your effort and distance in the days just prior to your century. Secondly, you will likely take a few days off to recover afterwards, so your weekly mileage may not change much at all.