I've had the interesting experience recently of receiving calls from a few prospective marathoners who are effectively looking to 'cram' for a marathon. One such aspiring marathoner called me last week hoping to complete the Paris Marathon on 4/15 and her longest run in the past month had been 5 miles...once!
Needless to say, this isn't exactly the recipe for success. In fact, it's likely the recipe for injury and disappointment. With this in mind, it's important to adhere to a training schedule (whether it's for 26.2, 13.1, etc.) that includes a thoughtful, gradual progression of increasing mileage.
The general rule of thumb around this is no more than a 10% increase from week to week. Assume you run 20 miles one week. The following week your mileage shouldn't be much more than 22 miles. Most quality training programs out there adhere to this general principle.
It's also important to recognize that your body needs a break every once in awhile. Increasing mileage 10% in perpetuity isn't the best idea. A better idea is to log 20 miles one week, 22 the following week, and 24 the next week. After increasing your mileage for a couple weeks, it would be a good idea to dial things back for a week to say 18.
This will give your body time to rest, recover, and gear up for the next bump in mileage!