Hi all - for what its worth, I've just completed the Paris marathon, which was my first, in 3hr 57 by following the programme in the book 'The non-runer's marathon trainer' by Whitsett, Dolgener and Kole.
It was a 16 week four runs a week programme, starting with distances as short as 3 miles and peaking at 18 miles a couple of weeks before the big day. I was a relative novice when it came to running and almost 15st at the start! I couldn't recommend it highly enough.