In reply to James Malloch:
Hi James,
I have just taken a look a look at the times for 2014 race, and a time of around 4.10 would put you in the top 5.
You cant compare one road marathon to another, trying to do it with trail marathons is even more pointless ( sorry to sound rude ), I have run the same trail marathon 3 times and although the route has been the same ever year the weather leading up to the race change it from hard packed trails (almost road compression) to spongy but not wet.
As for training, getting out 2/3 times a week sounds good and do a long run every other weekend.
You will want to do at least one run a week on trails, due to the shorter daylight hours make sure you have a good head torch ( I recommend any of the Petzl range )
The best thing to do is get out, do some jogging, do your long run of 20 miles at least twice leaving a month between them. But more importantly come race day, Enjoy it.
Hope that might help in some little way.
Ashley.
P.S The bloke that came second in 2014, Ricky Lightfoot ran a time of just under 4 hours ( has a road marathon time of under 2h 30m. ) just so that you can get an idea for how much slower the elites are going compared to their road times.