As slow as you need to go in order to get in the mileage.
First get the mileage under your belt, then you can think about pace. I'd leave the watch at home except for an occasional fitness check- run at the same solid, strong (but not overly straining) aerobic effort. As you get fitter, the pace this effort yields will quicken.
If you're up in the 90s or 100s for the first time, though, your body is going to need you to go very easy until you get used to it. That's going to be different for everybody- but just go as slow as you need to in order to get the miles in, at first. Once you're used to the miles, the pace will naturally come down.