Danster999 - I'm in the same boat as you, first marathon for me too, my brother talked me into a 10k last year and now I've signed up to Snowdon!
TRex - thanks for your advice, exactly what others have said to me, all about getting the endurance, miles, experience and hills. Going to work on base fitness first along with a few runs, then halfs and perhaps Surrey Slog and Bewl15 in July and keep going in that same way.
I am shit scared but so excited!
To read all the experience on this thread really helps, cheers guys, give me a push if you see me flagging in Snowdon!