I did this last year: Royal Parks half in October and FLM in April this year. As I had never run a full marathon, and my half training didn't include many long runs beyond 8 miles, I was keen to lengthen my runs more than anything else. I spent November and December just building slow miles.
My half times were around 1:38 and I wanted to aim at a 3:30 marathon, or 8 min miles, based on that. Guidance I read suggested long run pace to be about 8:50 to 9:15 min miles, to promote fat burning, which I stuck to but found very hard at first as it felt unnaturally slow.
Unfortunately, by the time I was ready to add back in the faster runs, i found it very difficult to pick up the pace again! Ended up doing 4:04 for FLM, partly because I was carrying an injury, but regardless 3:30 was never going to happen despite sticking to my plan.
What would I do this year? I would maintain a good long slow run at weekends so that I know I can knock off 14 miles or or so. I might alternate say 14 miles one week then 8 miles the next for this run. Mid week I'd stick with the 6 milers as others have suggested, one as a tempo, one as a bit of recovery. I would also stretch routinely to build flexibility, work on my core as I have a dodgy back, and do various static exercises to build strength in my calves and glutes which have caused injuries in the past, like shin splints.
I'm jealous! - I really enjoyed the build up in training and the Marathon itself - such a great sense of achievement after every long run, and completing the Marathon; well, I can't really believe it now.