Breastfeeding and going back to work

I don't go back to work until the end of June but I'm worried about the fact that I want her to still get as much breastmilk as possible. So right now I get 3 oz every time I pump and I pump about 3 times a day. I use the breastmilk for when we go out but I'm concerned about when I go back to work that I won't have enough breastmilk. Any suggestions other then to pump more cuz I literally have a hard enough time trying to pump 3 times a day. When I go back to work I will pump for 20 min every 2 hours so that's 4 times at work.