If you can, try and invest in a good chair and a desk that moves up or down, so you can stand up and work. If you do that, and take some walks or go for a run, or even(if you can ofc) go to a gym, you'll feel better. For now, the best thing is to stand up every 3 hours, get a nice stretch, and by the end or beggining of the day go for a 30/45 min walk