I would definitely agree with the others, train through it. You're not going to get a performance you're happy with if you push for time no matter what you do in the next month so run without any time goals. In fact, consider walking breaks.
As for what to do between now and then, I wouldn't try to jump right into the 20 milers. If you're really sold on needing a 20 miler, something I don't think is necessary physically but may be a mental pick-up, build up more gradually. Something like 18/20 would make more sense to me than jumping straight from 16 to back to back weekends with a 20 miler. Otherwise, realize that some training plans max out at 16-18 miles for the long run. In fact, from what I've been told, it's normal for European training plans to max out at 18.6 miles because that's a nice round number in the metric scale at 30K. 20 miles isn't a magical distance, it's just a round number.