Less whinging, more writing

I'm having a small dilemma. Normally at about this time (or sometimes sooner) I have my idea for Nano (http://www.nanowrimo.org/). Ideas are not my problem. I have bucket-loads, and my idea for Nano this year is an absolute blinder. I love it. It is the Christmas Eve of ideas. My problem is, you see, I love it a bit too much and I want to write it immediately. Now, I can see all you normal people out there scratching your heads and thinking, just write it now then. This is a non-problem, quit your whinging and get typing. Hmmmm. Good point, duly noted. I guess the problem really is that I'm scared. Mostly of two things. Thing one; that when I go to write it my story loses some (all) of it's brilliance and turns into a great big pile of poo. This is ridiculous. I am (at worst) a competent writer. I can string a sentence together. I know the rules. I've knocked out some half decent short stories in my time. I can probably do this. Thing two; I have never ever ever managed to write more than twenty thousand consecutive words and not get bored with the whole business ever ever ever apart from during Nano. What if my wonder story (which is a trilogy in my head) doesn't make it past the 10k mark? So when you take thing one and add thing two you get a great big bowl of irrational fear. I know I'm being stupid, which kind of makes it worse. It's like when I'm in hospital and they can't get the needle for the IV in and I cry. It's stupid. It doesn't really hurt and I've done far worse things to myself on purpose (hello foot tattoo and nipple piercings) and I know that getting upset does no one any favours, but yet I can't stop it. I guess my question really is, then, do I start to write now because I really want to? Or do I wait until November when I think I have more chance of sticking with it? Decisions, decisions... (if you want to, you can read more about my award-winning ideas and gold-standard procrastination by going here http://winningwords.org.uk/blog/ where I have been moon-blogging)