Hope this helps.
Regarding long runs, I don't think you need to do anything beyond 18. If it's flatter where you are then the pace will probably be much faster, so the fitness you get will be spread out over a longer time at a slower pace on the day (that's my theory and I'm clinging to it!). Definitely pace yourself. Try to get to Greendale at least in decent shape!
Regarding water, Wasdale has lots of water on the route (streams) so there's plenty of opportunities to re-fill a 500ml bottle.
I'm not saying that's advice to win it, I'm afraid, but I hope it helps you get round.