If you still can't express enough, you can use formula to supplement till you get back home.
Always try to breastfeed as much as you can.

Don't be discouraged yet.
Some tips you may need:
Ensuring that you are comfortable enough to spend time expressing. If you are not, you won't last. Secondly, if you are not well hydrated and well rested, volume produced may be reduced. Thirdly, after using your breast milk pump, use your hands which must be clean and express in a downward fashion. Finally, timing is important, expressing after a feed or just before the next feed works wonders. Also expressing at night too after breast feeding. Be determined to give your baby the best.