You really only need enough to get you through the first day or so... then do your best to pump regularly at work and your baby will eat what you pump one day the next day.
Don't stress you WILL get more milk when you pump at work because you won't be feeding your baby. Try to pump at a usual feeding time, and you'll be surprised how much you get.
That business about not being able to pump after 3 months just isn't true for ANYONE I HAVE EVER KNOWN. Most women don't pump unless they have to be away from baby, and many don't run into that until 3 to 4 months. Relax, you'll do fine.
Pumping a day ahead is actually much better than stockpiling. Your breastmilk changes over time to meet the needs of a growing baby, so the more recent the pump, the better.
Save what you can over the next couple of weeks, then just be really serious about good pumping at work. Make sure you have an uninterupted half hour, some place quiet and safe, and drink plenty of water.
Good luck mama.
HTH
T.