I tried just building mileage for a while and running at threshold level but that didn't get me too far. It really depends on what your goals are.
I ran 90 mile weeks with most of it around 7:00 pace. Three times a week I would pick up the pace to insert quality but could only get myself up to 6:00-6:15 pace on those tempo efforts( progressive, fast finish, or steady state throughout.
I didn't have the turnover for anything under 6:00 pace. And it wasn't just the turnover, I was also gassed aerobically.