OK, lets back up a bit and ask why you felt you needed to "impose" (that's the term for shuffling into printer's spread order) the document? Are you printing this yourself? If so, then use File > Print Booklet... or use an imposition script (there are several) or plugin like IDImposer (Overview | IDImposer). If this is going out to print, imposition is the printer's job, not yours, and you don't help anyone by doing it.
Exactly, Peter! If you're sending this to the printer send it in single pages from page 1 to page 20 in the correct reading order.
File>Print Booklet for printing yourself (you will need to print page 1,2,19,20 separately, then 3-18 seperately, then wrap the 1,2,19,20 around 3-18 to finish)
This is related to a question I have. Is there a way either in InDesign or Acrobat to make the exported PDF automatically update page numbers as the pages are shuffled around?
If you no more need association with master pages after the forced shuffle:
1. Select all the pages in the Pages panel and from flyout select override master page items and select "detach" from master
2. Delete the auto page numbering from master
3. Shuffle the pages!