Miles. If you do 100 miles/week total for the last few weeks before the taper week preceding your century, you'll be able to do the ride. For a fast century, 200 miles/week is good.
Rather than change the pace during a ride, it will help you more now to change the pace for the whole loop. IOW, ride it as hard as you can once a week, easy once a week, and at the pace you intend to use on your century once a week. And as much other riding as you can manage. When you ride it hard, your breathing should be very fast and deep the whole way, or at least try to become able to ride that far at that effort.