Pretty much what everyone is saying so far is spot-on: just increase a little at a time. Your regular rides during the week might actually be the same distances, but on the weekends, increase the mileage of your "long" ride a little. You might even throw in a mid-week long ride. It might look like this:
- Mon - Rest day, off the bike
- Tue - Regular ride
- Wed - Longer than a regular ride
- Thu - Recovery ride
- Fri - Easy & short ride
- Sat - Longest ride of the week
- Sun - Recovery ride, spinning fast to help the legs recover
It's also important to ride as close to everyday as possible. Increasing the mileage of your "long" ride is good, but riding everyday is additionally good.
Last, if you can do this, it's important to get in as many "long" rides as possible before your target ride. That way, you'll know how to hydrate & eat for it, how to pace during it, and completing it will actually be fun instead of a challenge that leaves you physically shot for a day or two as your body recovers from the effort.