In reply to James1982:
I asked the same question last year
http://www.ukclimbing.com/forums/t.php?t=481481&v=1#x6634945
and got some good advice. (identical thread title: great minds and all that)
Given my abject failure in my target marathon last weekend, I'm probably the last person you want advice from however...
My guess is if your goal is sub four hours you don't need to change anything until the start of your sixteen weeks. If you want to go quicker...
I had an easy summer, just running when I wanted to - but rarely less than about 20 per week. I then did a solid block of high mileage weeks in October and November. Around 50 per week, but all from of two short runs five days per week, because this fitted around work and homelife. I then through December I reduced the number of runs, and mileage, but introduced a few longer ones and a few speed sessions. This meant that at the start of my 16 weeks I found I could go straight into 50 and then 60 miles weeks (possibly a bit over the top) with midweek speed sessions and weekend 18+ long runs without having to build up to it.