I also work in an office for 8-9 hours a day. Before I leave for work I walk my dog which is only about 5-10 minutes. Also I know it seems like working out takes a long time but it really doesnt. I always feel like I don't have time to workout after work but that is just an excuse to me now. I tell myself every time I don't feel like running that it only takes 30 minutes. 30 minutes that will help me in the long run and keep me in shape.
If you still can't do that then there are some things that I have found that work for me. If you have a lunch break take a walk and eat lunch outside or just walk the full lunch break and eat at your desk while you work. I have a private bathroom at my job so I do squats every time I go to the bathroom which is a lot because I force myself to drink at least 64 oz of water just at work.
I hope that helps a little.