I agree with cjrt about electing to go out at 3:40 pace. But the purpose of long runs in the 20-mile range is that they teach your body how to burn stored energy in miserly fashion. Since your longest run was only 15 miles I am very concerned about what awaits you from 20 miles on. To be frank, that last 10K is going hurt, and you'll probably be forced to a shuffle or a walk.
