Ditto what everyone else has said really - time on your feet and positive mental attitude got me round the high peak marathon (40ish miles), whereas my longest training run was probably only 20-25 in one go (and that was with a hungover team mate!). Just keep smiling - if you think you can do it, you can. Also think of the massive bragging rights it earns you! (generally a big incentive for me! ) Good luck and enjoy it!