Fields are quite simple, but to make them easy to work with you should add a
personal toolbar to Word and put on it a couple of buttons to call a pair of
macros:

Sub FieldCodes()
   ActiveWindow.View.ShowFieldCodes = Not ActiveWindow.View.ShowFieldCodes
   Application.DisplayStatusBar = True
End Sub

(there is a keyboard equivament to this - ALT+F9)

The other is the example code at http://www.gmayor.com/installing_macro.htm
which explains how to add macros
and that macro has keyboard equivalents (CTRL+A then F9, but does not work
when the document has headers with fields)

The step you appear to be missing is the updating of the field

Press CTRL+F9 - this gives you a pair of brackets {}
Between them type = CTRL+F9
so you get {={}}
Between the second pair of brackets type Numpages and outside the bracket
type * 26 thus:

{ ={Numpages} * 26 }

Toggle the display - the field will be blank. Hit F9 or the toolbar button
and the display will show the page number * 26.