If you pause the movie, you can resume it anytime, even past the expiration period:
The catch is you can't use the iDevice for anything else until you finish watching the film.
Sorry, but there is at present no way to extend a rental nor to re-rent without downloading the movie again. You can suggest such a feature to Apple via their feedback page, if you wish:
but whether they ever will, or even in practicality can, implement that I can't say.