I'm no expert Sara, having been running only a year or so, but the standard format for novice marathon training is to do 3 relatively short runs in midweek, and a long run at the weekend, with a fifth, optional, 'cross-training' day when you do something else, even if that's just a good walk or a visit to the gym.
The programme I followed started at Week 1 with 3 runs of 3 miles each and a 6 mile run at the weekend, and graduated to runs of 5,10, 5 and 20 in Week 15, after which the taper began. Every 2nd or 3rd week between the 1st and 15th week was a 'stepback' week when the long run would be shorter than the previous one. So the long runs went in a sequence of: 6,7,5...9,10,7... 12,13,10... 15,16,12... 18,14,20... 12,8,26
Go to this link, and read a bit about it!
http://www.halhigdon.com/marathon/Mar00novice.htm
Andy