Well done Steve! I started running in January this year and like you I am taking part in my first Marathon this year!
I found having a target was a great way to gain motivation, its even more motivating when your doing it for a charity as you don't want to let them down either. This forum is also a great way of staying motivated because everyone here encourages you to do well
As its already been said, take it steady and Im sure you will be fine. Don't aim to finish the marathon in a set time, just aim to finish it because finishing it in itself is a brilliant achievement!
Here is a 12 week training program from the Edinburgh marathon website :
http://www.edinburgh-marathon.com/do.../beginners.pdf
or visit:
http://www.edinburgh-marathon.com/?training-programmes
Hope that helps
O.