MM, I went back to work when DS was a similar age - but only two days a week (albeit one was a very long day and I was away from him for 12 hours).
I expressed for him till he was a year old and he built up his food intake steadily while I wasn't with him. He didn't eat much when I first went back to work, but did drink quite a lot of EBM.
I would say start expressing now. Definitely. I didn't build up a supply in the freezer before I went back, which in hindsight was really daft. If I had, it would've been a breeze. But I expressed at work, and played catch-up. I would leave 20oz on my 12 hours away from him and less when I was away from him for 10 hours, because I worked 3.30 to 11.30, so I could feed him more before I left for work.
But DS was a huge fan of EBM and not a great fan of food to begin with - once he got the hang of eating, he didn't need as much milk. And when he was about 10.5 months, I gave him half and half EBM and cow's milk, because he was getting so much bmilk when I was with him, I didn't think it would make much difference.
But definitely start expressing now. Try expressing as you feed in the morning, if you can? Don't freeze it in huge amounts though - I used to do 1, 2 and 3oz amounts in Lansinoh bags, so that it was possible to make up bigger feeds. DS took about 6oz a feed.
HTH a bit - and it will be easier than you think because your baby's adaptable. I promise!