Start with the number miles you'll be happy and comfortable with. Throw in some short sprints or bursts whenever you feel like it up the hills. Overall, it should be at an enjoyable pace, and consistent throughout the week. The importance should be placed on how well you get creative with exploration, rather than sticking with the same route day in day out. If you want to increase your mileage from there, don't raise it any more than 10 % per week (note, this is a maximum, optimally have it at a 5-10 increase per week).
Easy way for that is to make it part of your lifestyle. Go to work on your bike. Cycle to the library. Shopping works fantastic as well. Enjoy yaself out there. :D