I recently did 13.1 miles in less than 90 minutes as a long run, and I didn't really feel that tired or sore at the end. I probably could have run closer to 87 or 88 minutes if I pushed a little more.
That 13.1 miles was the furthest I've ever run. How fast do you think my marathon would be? How about if I trained specifically for a marathon for 3 months?