Your PT is right in one respect. Sitting at a desk will make your knee stiff. Even if you can't be up and walking around all the time, do try to have a little walk about once every hour.
You did go back to work a bit early and your knee will probably react by getting a bit more stiff and swollen. When you come home, try to spend extra time resting, incing and elevating.
Now you're back at work, ordinary life should be your PT. Continuing to do PT as well as working could be a bit too much for your knee. You won't lose the ROM you have already achieved, and this ROM can continue to increase for a year or more after your TKR.
You did go back to work a bit early and your knee will probably react by getting a bit more stiff and swollen. When you come home, try to spend extra time resting, incing and elevating.
Now you're back at work, ordinary life should be your PT. Continuing to do PT as well as working could be a bit too much for your knee. You won't lose the ROM you have already achieved, and this ROM can continue to increase for a year or more after your TKR.
New Zealand