In all honesty, if you're both new to the sport and only interested in fitness, I would just ride lots. Try to vary your terrain and pace, but at this point just go out and have fun. Ride as many days as you can, and remember to leave enough time for recovery and adaptation. You should find that, at this point, without any sort of structured training, you will see some marked improvements pretty quickly.
If you find after you've developed a base level of fitness that you want to build a structured training plan, I would look into Joe Friel's "Training Bible," Chris Carmichael's "Ultimate Ride," and (especially) Greg LeMond's "Complete Book of Bicycling." Each of these books has some great info on what you need to consider when constructing a training schedule.