In reply to Shaso:
On the Garmin watches, at least the posher ones, there is a PacePro function which I have used once on an Ultra to try to slow myself down at the beginning. It takes your expected time, considers the splits and asks whether you go fast or slow up hills. The race didn't go that well (I think I need to accept that I am a positive split kind of runner and I was trying to fight that) but what it was suggesting felt about right in terms of differences in pace on hills, descents and on the flat.
On the web when planning this, the possible paces were ridiculously fast for me and the terrain, but once on the watch I could alter that to meet my needs to stay on track.
This may be irrelevant, but it looks as if the tech is out there to try to optimise peoples pace on these vary variable races.