Is there any way you could ride your bike to work, even part way? Commuting is the absolute best way to incorporate riding into your everyday routine. Even if you commute is short, time in the saddle is what counts.

There are some century training threads somewhere in here, take a look at the New Riders section (?). Being consistent is more important than piling on the miles closer to the century. Get riding regularly now, and increase distance on weekends (gradually) ought to get you there. Of course, I don't know what condition you are in now...I'm sure others will have good (better!) advice than I can give.