The problem with trying to work out HM pace from intervals is that having done so little mileage you're unlikely to have a close relationship between what might approximate your 3k/5k pace - i.e. what you can manage for 800 intervals - and what you could maintain over 13 miles.
I'd go with BDB's estimate. The 800 intervals on paper suggest you've got the speed to go faster but your endurance would probably let you down if you went off too quick. If you're feeling ridiculously comfortable with a few miles to go, ramp it up a notch.
Edited: 12/08/2010 at 15:37