The purpose of the the long slow distance run is to acclimate your body to being in motion for longer than it normally would be. Run slow. A 12:00 or 13:00min/mile pace is 100% allowed. Stick to flatland as much as possible. Hotshot crews will regularly run 8+ miles during their Long Slow Distance runs, so you should be shooting for similar mileage. A good rule of thumb is not to increase the distance of your LSD runs by more than a mile at a time.