Many thanks for all the replies.
Although I built up quickly, I did build up. In the last two weeks I did two 5 miles runs during the week then a 9 mile at the weekend followed by two 7 mile runs last week and then 12 miles last night. Maybe I've got away with it so far because I'm relatively young (29), however I don't want to push my luck so although I'm fine today (bar a little stiffness getting out of bed) I'll stick to 7 mile runs during the week and 9 at the weekend for a few weeks- give my body time to save up.
I take your point Dave, I suppose we are all products of the culture we live in and for me running a marathon is such a challenge that I never thought I'd be in a position to contemplate attempting. For the time being, I'm more concerned with rising to that challenge than developing as a runnner but I can see the logic of not throwing a 16 year old kid on in the champions league final no matter how skilfull he is.
I'm hoping that my cycling and running will complement each other and do intend on taking part in a couple of half marathon's early next year but for my first marathon I'm just really looking to get round it in a reasonble time, around the 3:30 mark. So with what you guys have said I'll just build up my mileage (albeit a little slower!) and maybe think about speed runs etc this time next year to improve from my first marathon time.
Many thanks again for all the replies and advice.
Cheers Ross 