Don't listen to the above guy... You can get there with lots of racing or none.. My Pr for the marathon (mid 2:2Xs) came with only 1 race in the previous 6 months (a 5k race for my company). I got sick the week I was supposed to do a half during my build up so I bailed. If you have the right training racing isn't necessary. You also don't need to be a stud at the half to run a good full... if you peak right your pace in the full should be about what you half pace would be 6-7 weeks out from your full (also a good build up strategy to race a half about that far out).
I would agree with other posters saying to try to get to 120+ mpw asap, but I would only do that if you are already accustom to 100mpw. I have found that milage really isn't likely to hurt me but speed faster than Marathon pace can when at high volume. Eat a lot sleep alot. and long runs are the most important thing.
the long term goal should be to have your long run be one of your workouts most weeks. Just a good progression to where the last 8-12 are cutting down to marathon pace. Practice fueling! You need way more fuel than most people think... if you can get 500-700 cal in by 35K you are pretty golden. the first half of a marathon is an eating competition. This doesn't mean you need to be taking 500cal in on your LR days... I usually do at most 300 cal and only if I am doing 23+ but you need to get you stomach used to fueling luckily you have sometime
Consistency is key.. I debuted at high 2:2Xs (as a mid 15 5k guy) I didn't have one workout longer than 8 miles of quality but had a couple/few years of 105-125 mpw in my legs with the last 4 months having 6+ 20+ mile long runs most of them were progressing to fast paces. While I do not think it's a good Idea to only have workouts of 8 miles of quality the point is consistency and long runs are what will take you to the next level not killer workouts.
Last thing.. as long as you are consistent it really doesn't take that many weeks of 120+ to get you into killer marathon shape given a the previous weeks were around 100... It also doesn't take too many more 120+ mile weeks to destroy your body so be careful.. take down weeks when needed (90-105mpw)