Came 311 out of 1,100 in 4.00.27. Was a bit miffed about not breaking 4hrs, which was sort of a target, though I didn't really know how I was going to do.
Got stuck behind a line of slow / knackered runners on the final descent, when I was flying. Would easily have broken 4.00 if not for that, but hey, it's only a number ...
Incredible support, that is the main thing that sticks in my mind: Tour de France style cowbells and 'Allez! Allez!' and 'Bravo!'. Hundreds of people along the route - despite it being in high mountain country.
Just goes to show how much people appreciate / admire mountain sports over there compared to here, where it's seen a strange eccentricity.
one factor that I don't think has been mentioned is weather: I find you need to take on a lot more calories in cold conditions to keep energy levels up.
My worst bonk ever was on a long, cold winter run round the surrey hills.
On the subject of uphill / downhill, I find differently to Mick W that in fellraces the ups are the opportunities to fiddle around in the bumbag and get some snacks down - but that's because the gradients are so steep and long that you slow down to more or less a walk (in AL cat. races anyway).
In lower level trail running where you run most of the inclines that's less easy.