Offhand it sounds like you most likely have good fitness and a reasonable amount of time in the saddle. The key challenges here will be climbing ability and time in the saddle.
If you really can't find hills, consider getting an indoor trainer with variable resistance. This will not only get you used to the efforts, it will result in a more effective use of your training time.
Saddle time won't be easy given your time constraints. Getting used to those durations is as much mental as it is physical. It's not just fitness, it's knowing hydration and nutrition, including what you can tolerate eating and drinking after mile 101, how to pace your resources and so forth.
If you have a penchant for stiff and responsive bikes, you may find that doing 80, 90, 100 miles on a racing-style bike might work out -- but might not have sufficient gearing, and may leave you feeling fatigued and sore by the end of the first day, and punishing on the second. You may need to cushion the bike (e.g. wider tires, better saddle, more upright position, wider cassette) or possibly look into a cushier bike that doesn't detract much from performance (e.g. endurance or sport touring bike).
Since the ride is in January, I recommend you schedule a hilly century for September or October. That will help shake out your setup, as well as test hydration and nutrition