Good shoes are an asset for u now. I worked 8 hour shifts on mi feet rite up to 20 weeks. It is hard but the shoes and putting your legs up on breaks does help. Good luck
Transitioning from a job where you didnt have to be on your feet all the time to one where you do will be tough on anyone. I will say this though, i remained very active in my pregnancy up until 34 weeks and i get cramps if i walk too much ): i feel like a potato now lol but yeah invest in some good shoes! They really do make the difference, i promise it gets better!
Oh and I forgot to say that I'm 18 weeks 4 days now so I can tell you from my personal experience that your body will tell you if you're over doing it
I'm a manager at a super target and work 9-10 hour days and on my feet for most of the day. You're body would be feeling that way even if you weren't pregnant since it's new being on your feet the whole time. Because your are pregnant just make sure that you wear good shoes and sit down and put your feet up when you get a break. 4-6 hour shifts should be just fine for you for most of the pregnancy. Good luck and congratulations!!