All my weekend runs tend to be in the 10 to 20 mile bracket and always, always in the hills. So I guess my natural pace is geared to that and, for long races, all I do is set off pretty much at my same old 'speed'. In the rare occasion of doing a short race, I then speed up as best I can. Having done some ultras though, the last thing I would say is that you need to train running slow and again I just run at my default hilly run speed which is something like 5 mph on average. In last years fellsman I think I ran the first 20 miles to Dent in something like 4:30 hours and that included going up and down Ingleborough, Whernside and Gragareth/Great Coum on the way - all in the next 40 odd miles maybe took me a further 13 hours of running, what with food stops and stuff so clearly weariness, time on my feet and distance kicked in but I really don't think doing the first 20 miles slower would have made any improvement to my final time; in fact I think it would have just made it proprtionately that much slower. Its all about having a natural metronome like pace and trying to stick to it best you can I think.