Couple of thoughts
1, Does 140 pages print have to be 140 pages of ebook?
I mean by this if say you had 10 chapters X 14 pages each, it might be more logical and easier for the user to make each chapter an ebook page. This will certainly cut on the cut'n'paste
2. This is very vague, but I think another trick might be to use a word macro. The macro does not have to be perfect, but if it does a first pass, then you can manually clean up whatever "mistakes" it generates
Word basically includes a very powerful (but complicated) programming language. A true Word expert would probably figure out from scratch how to do this, but here's a cheat that I would try
See if you can identify a sequence of key strokes / menus that slices off one chunk/page and fixes the links. I imagine this would be some combination of find/replace, jumping around the downs, arrow keys etc. It might easier to figure it to slice off the last chunk of the big document.
Once you have these keys figured, use the macro recorder to record a macro of these key strokes (try to avoid using the mouse completely as this doesn't work real well). Word will actually write a little program based on the keys you pressed. When you have finished your macro, call it something like SliceFromHereDown or SliceOneChunk etc. and assign it to a key.
Then just hit that key as many times as necessary.
***IMPORTANT*** Obviously as there are lots of ways that this could go wrong, make a COPY of your original file, and do your practise runs and slicing on that.
Hope this helps.
By the way there is a pretty good word forum atwww.experts-exchange.com(I use that site myself sometimes to get answers to these types of questions) and if you ask a question there, there's a pretty good chance somebody will know the word macro language and be able to give you more precise pointers.