As I re-read the original post, another point. If the idea of 20 is to get into the mindset that you can go the distance, the original poster really doesn't need that final one. I see an 18.5 (conveniently, very close to 30K), a 20, and a 22 already in the books, as well as a pretty nice pair of races on Labor Day weekend that totals nearly the full marathon distance of racing in a 3 day period. I hope the original poster doesn't need another 20 miler to wrap his mind around the idea of going the distance.
Not saying that nobody needs this mental boost but not in this case and not with a half marathon in the plan. I'd race the half and do about 2 for a warmup (my usual pre-race warmup) and 1-2 for a cooldown.