A.C.
I went back to work--which included multi night business trips--when my daughter was 10 weeks only and managed to keep her exclusively on breastmilk until she was almost 9 months old when she started showing interest in solid foods.
I would definitely pump before nursing in the morning and try to get in 1-2 sessions during the work day. If your place of employment doesn't have suitable facilities for pumping in a clean environment, pumping and dumping will help keep your supply up. I had to do this a lot when travelling when I didn't have clean facilities or a place to store milk over several days.
I also sometimes would nurse on one side and pump the other--then switch. I seemed to get more out of the pumped side when I was nursing the other.
While I am a huge proponent of exclusively breastfeeding as long as humanly possibly AND doing everything you can to secure it...formula isn't poison. Do what you can within your means to keep up with the breastmilk and then use some formula.