Your third and final 20-miler. For the first and second 20-milers, I suggested a pace 90 and 60 or more seconds slower than race pace. But in all honesty, it is not that important for you to run this workout faster than the previous 20-milers. There are too many variables, including your level of fatigue toward the end of a demanding training schedule. Most important is the fact that you were able to run three workouts this length within a space of five weeks.
Some coaches recommend just that, even taking runners beyond marathon distance.
I don't necessarily disagree with this approach, but since you signed up to follow my program, you'll have to peak at 20, not farther. The most important point of any training program is the totality of that program, and the results it brings. A flash speed workout with quick splits may look good in your training diary, but it could bring you to the edge of overtraining. The same with running the long runs too hard. Your time in the final 20-miler won't count three weeks later.