I was going to mention the mental side, but I'm not sure if that's something you can prepare, or even get through experience I have seen people just "crack" and retire, even though there's probably not too much wrong with them what I try not to do is think about how much is left to the finish, just about the next section to the next CP...
if you've recce'd the course though, you should have a good idea of the "extent" of the challenge, so it shouldn't come as so much of a mental "shock" it also help with pacing (one of my points)
what cougie said - if you're expecting to DNF you obviously haven't trained properly that said, if it's a mega-long one that you haven't done before, you can't know exactly how it's going to go, so there's always a bit of a frisson on the start line - i.e. be nervous, but not scared witless
the only race I've ever been scared at the start of was the Kielder Borderer a few years ago, but that was because I was completely unfamiliar with the area and there was snow on the ground and the clag was down (and there were only 30-odd entrants and they all had north-east accents except me)
Sun 28 Sept - Swinton 10 - GBM, FOAR Sun 05 Oct - Congleton Half - GBM, Sat 11 Oct - XC Woodbank Park - GBM, Sun 19 Oct - Green Drive 5 (Lytham) - Daz Sat 25 Oct - Halloween Horror at Rivington 10k (Headtorches Req) - GBM, Sat 01 Nov - Langley 7 - GBM, Sat 08 Nov - XC St Helens - GBM, Sun 16 Nov - Preston 10 - GBM, Sun 23 Nov - Barnsley 10k - GBM, Eric, M, Sun 30 Nov - Famous Grouse FR - GBM, Sat 06 Dec - XC Boggart Hole Clough - GBM, Sat 13 Dec - Festive Fun at Delamere 10k (Headtorches Req) - GBM,
if anybody fancies more of a navigational challenge, there's a new series of 3 hour score "O" events that I'm looking at doing some of: http://www.explorerevents.co.uk/race-info/ (I happen to know the locations are Edale, Glossop, Totley and Goyt Valley respectively)