You should do a decent amount of goal pace running. Once a week during marathon training is good. I've taken two approaches: folding 8-12 miles of MP running into my 20 mile long run (I usually do 3 twenty mile runs in my build-up), or doing a separate MP run each week in addition to the long run. I include interval workouts throughout, with the intervals getting longer as the training plan progresses.
With the MP during long run strategy, I usually include an interval workout that week, and I only do the MP/long run every other week. On the intervening weeks when I don't fold MP into the long run, I do a moderately-paced long run, an interval workout, and if I'm feeling good, a tempo. Longs runs with MP are taxing, so you need to give yourself time for recovery.
With MP separate from the long run, I do an interval workout, take one easy day, then fold MP into a longish run 11-15 miles. I do anywhere from 7-10 miles at MP with a warm-up and cool-down. Sometimes, I break the MP into long chunks (2 X 4 miles, 3 X 3 miles, 1 X 5 miles). Then I take a two-day break before doing a moderate pace long run. The one-day break between workouts means that you do the MP runs on tired legs.