I went back a little later than you - 19 weeks. The time to start a frozen stash of milk is now. Freeze it in one, two and three ounce bags. If you can only manage to express half an ounce now and then, keep the half ounce in the fridge until it's up to an ounce or two.
I don't think you can ever have too much of a stash. Frozen milk keeps for a year, and in the fridge it will keep for days too. I found it very hard to get my head around the fact that breastmilk is not cowsmilk, it doesn't go 'off' as quickly and is pretty amazing stuff in terms of how long it keeps fresh.
At work I would express three times a day, morning, lunch and late afternoon. I also expressed at 9.00 p.m., 2.00 a.m. and 6.00 a.m. DD had two four ounce bottles during the day whilst I was at work. I would just put it in the work fridge, or keep it in an insulated bag. (I am away from her for 12 hours - 6.30 a.m. - 6.30 p.m.)
It does get easier around 6 months, just take one day at a time. I worked full time from 19 weeks, and breastfed until 15 months without having to give any formula.
You may be lucky and have a baby that does reverse-cycling, that is takes all the food it needs from you during the evening/night time and doesnt' need so much during the day.
One more tip - cosleeping is a lifesaver if you have to work and breastfeed. It meant we all got enough sleep.