MrC - I agree with MM that take double halfM time and add 15 minutes gives approx marathon time, so based off 1.30 half I'd be disappointed with outside 3.15/3.20.
My half pb is 1.36,set 2 weeks after London where I ran 3.31.
I'm aiming for 1.30 half next year (did the GSR 10miler in 69 minutes) so it should be possible and a 3.20 marathon.
Read about the Yasso training , not tried it but for marathon training I do 800m repeats at 10k pace (so 800m in 3m20) with 200m recoveries which is tantamount to the same thing.Also do 1k,1mile and 2 mile reps at 10k-10mile pace.
To follow up Mark H build up to London my last 12 weeks my long runs were : 12,10,14.5,17,20,20 (race),13 (race),22,13 (race),22,17,10,FLM.
Not sure this helps anyone , but its good to exchange views.
As a postscript, I only do 1 marathon a year but this autumn/winter I'm maintaing a long run of 12-16 miles every other week to keep the endurance base.