I can only say that I did "train for a century" by riding only 3 times per week. But, I got lucky with the weather and people let me draft them and it was a flattish century (only 2400 feet total elevation change). Boy, was I tired after the century. Those last 30 miles seemed to take forever.
I would advise increasing your speed a little bit more and to do at least 40miles 2-3 times per week including hills. because otherwise the century will take you too long.
2014 Liv Lust
2013 Specialized Fate Expert with carbon wheelset (sold)
2012 Specialized Amira Elite
2010 Santa Cruz Juliana with R kit and Crampon pedals (sold)
2011 Specialized Ariel Sport,suspension post,Serfas Rx Women's Microfiber saddle (sold)