Here's a complete hypothetical for those that like numbers. I use a formula that predicts a person's ideal marathon time for a specific day and course. You input your first half time and final time, and the formula will spit out what you could have run on that day and that course if you'd run perfect splits. Kind of a masochistic thing that tells you what you could have done that day if you hadn't been stupid and gone out too fast

It's completely unscientific and hypothetical, but uses the ratio of every second that you run too fast over the first half, you give back two seconds the second half. I've plugged dozens of marathon performances into this and it is surprisingly accurate. Most people see their "ideal" number and say 'yeah, that's the shape I thought I was in if I ran smart'.
I can't remember where it came from originally, but the theory claimed that a negative split would contain the same ratio. Running negative split would cause you to leave time on the course in the same manner as going out too fast. Plugging Hall's numbers in you get 2:07:50.