Here is a real world example from a mediocre rec runner. I ran a half marathon in 1:31 (7:00 pace) two weeks before starting a 18 week marathon training cycle. I then proceeded to complete my marathon cycle, averaging 45 miles per week. At the end of training, I was confident that I could average 7:40 pace for the full marathon, even running a 22-miler at 7:30 pace with negative splits. I proceeded to run 3:40 (8:30 pace) for the marathon.
Lesson learned, run more than 45 miles per week. To answer your question more specifically, I would think you should be in good enough shape to run 20 miles at your goal pace before starting your marathon cycle. Might sound extreme, but it ensures you have enough of a base to do the mileage and marathon specific training to run an evenly paced marathon.