Lots of great advice from wiser people than me, but anyway here's my experience for what it's worth!
I agree with Johnny J, for many people, 4 training days a week for a marathon is plenty.
For this year's FLM I trained 15 weeks, 5 times a week, with a longest run of 22 miles. Ultimately, I picked up an injury in the last few weeks, had a bad and painful race. I couldn't run again until over 10 weeks after the event and spent quite a bit of cash on physios.
I ran this year's New York on the back of 14 weeks of training, running 4 days a week. My average weekly total was 30 miles, maximum weekly total 42 (at 5 weeks out). The longest runs were 2x18 miles and 2x20 miles with shorter weeks in between.
My week consisted of a speed/fartlek session, a hill/tempo session, a mid-long run at moderate pace (often broken up with walking) and a long run on Sundays. I also did a weekly yoga class until the last 5 weeks, when that became a rest day. I never ran more than 2 days in succession and never did hard days back-to-back.
What about the times? London: 4.16.50. New York: 4.11.59. And after just a week of I've already started light running again. To improve I know that eventually I'll have to add a few more miles, but I've learned through painful experience that the key is to do it very carefully and gradually.
Good luck from me too.