To just finish you could get by with 40-45/miles per week - your target time for a first timer is about 4:00 hours. Without a good running background or quality training build-up...3:00 - 3:30 would be a real long shot unless your the gifted type.
I tend to run 3-marathons/year and I am a 3:10 runner in a marathon only event on a pretty flat course like MCM in good weather conditions. I get there on 65/miles per week on a 8-12 week cycle peaking at near 90. I would recomend the book Advanced Marathoning as it will teach you all the necessary components you need to include in your training.

My training will change greatly depending on the event and the time of year. As an example - how I train for Chicago, Detroit or Disney (Flat and Fast) is very different that how I train for Boston or Pikes Peak which have very different demands.

There are 2-schools of thought on the need for long runs and the Hansons have proved you don't need the long run to be fast or finish strong in the marathon. In fact their longest run is near 16.

The other (and the most followed) is the need for some long run training. I believe the long run is important as I tend to run (4-5) 20-mile runs in a given build-up, along with at least 6-other types of run specific training sessions (such as speed work, general aerobic, tempo, short, middle-distance, hill repeats, etc...). Although I have run a few 3:20's on ZERO long runs and on 40/miles per week.
Then again - I have run about 16-marathons since January of 2000 and tend to peak and re-peak year round so I can get away from it here and there.
Don't forget - Mother nature has alot to do with the outcome.

The bottom line is build or follow a quality training plan that has a reasonable goal. Taper and go into race day knowing your are fit and strong and are ready for the challange.