I'd be anchor - literally!
When I wrote the code for the schedule calculator on my site I had 18hrs as the fastest selectable time. My reasonings were that anyone going faster than this would most likely know what schedule they needed to keep to (at the time there were probably only 8 people with a time less than 18hrs anyway) and I didn't think the calculations would work at those paces. Also I didn't find Billy Bland's times until later.
I've since done a local version where I could drop down to 14hrs and the resulting times are pretty close to Billy's so it would seem that the slowing down over the course of the Round is reasonably consistent across all speeds/schedules though of course the fast guys are still quite a bit faster. Even when fresh I couldn't get up Yewbarrow as quickly as BB after 40 miles.