The important thing is to build up slowly - an increase of maybe 10% per week, with some rest weeks.
I agree with previous comment about one long, one medium and a couple of shorter, faster runs per week.
Depending on your level of core fitness, some cross training or Pilates would probably help.
Finally, don't forget the mental side. You want to ensure that your longest run in training is long enough, so that when you reach that point in the race and hit a mental wall, it's not too far to go. For me, before my first marathon, my longest run was about 35k. In the race it suddenly got very hard at that point. However I knew that I only had 7k to go. If my longest training run had only been, say, 30k, that extra 5k in pain may have been too much.