Swift Publisher is the better more mature application and not too expensive.
I got mine at a discount either with a bundle or at
http://www.mupromo.com
Are you able to saddle stitch all 5 sheets of paper inside each other? That sounds a little thick.
Any way this is the principle of imposition:
The pages in your case run from 01 to 20 and these face each other on the outside:
20 | 01 (outside)
to get the insides of this you swap them and add one on the left and subtract one on the right:
02 | 19 (inside)
Next set is:
18 | 03 (outside)
04 | 17 (inside)
and so on.
Knowing this you set up a template in *Page Layout mode* with a horizontal letter layout and add a floating text box for the left hand and right hand pages. Add another pair of floating box for the page numbers and number them according to the imposed numbering system above.
Do not use the Facing Pages option which is broke in Pages anyway. You are literally setting up the document in the manner in which it is finally printed.
Take all your text and paste it into the 01 Page floating text box (or cover). It will show an overflow at the bottom right of the box. Click on that and click on the next floating textbox in the real page order ie Page 02. This is where having pre numbered the pages will come in practice.
Thread the boxes with links in order. Fix up your type and styling and you are ready to print.
When printing work out how duplexing comes out of your printer and send it as needed. Or work with printing odds and evens to have them print correctly back to back.
This will be a recycleable template.
*Another method* is make a Pages document using normal half letterhead size pages and send that to a pdf.
Open the .pdf in Preview app and show the thumbnails side bar.
You can drag each thumbnail into a Pages template which has 2 ready made image place holders on a horizontal letter sized document. Drag each .pdf into the appropriate order as above.
Peter