I walked up and down Snowdon last year, it was my first big hike and built it up in my head into a mountainous challenge and got very worried about it and whether I'd be able to cope and what I'd do if I got into trouble.
I wanted to do it as an achievement for myself, something to show myself that after all the weight I'd lost and moderate fitness I'd gained during the year there was something 'solid' I could now achieve that was previously uncontemplateable.
I went with a very small group and guide, up the Pyg track and down Miners and thoughoughly enjoyed it, while it was hard going, in heavy rain / fog both ways and my thighs hurt for about a week after, the adrenaline had me bouncing on the way down.
BuzyG is absolutely right, have fun enjoy the walk and the occassion. Dont get too worked up about it (like I did). Let us know how you get on.