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.