Good question Carla. I had the same problem but I found some solutions. If you do not have much time during the middle of the week. You could try doing more activities on the weekend. Just try to get a group dance or walk. if you start doing these activities you will likely improve your physical condition and your performance at work. Why not get in some gym to gain some physical conditioning? If you like some kind of fight you could get into some group needs to fight as much physical effort and you lose a lot of calories.
During the week at work if you have a little rest time you could walk and stretch at least 3 times a day. It's important to know that the exercises don't only help to improve the physical condition of decreasing body fat while increasing productivity even during your turn. it's important to know what exercises diaries bring a spare time job and a diminution of stress and tiredness from work. So after walk and stretch you could do fast exercises as jump rope, pushups andclimbing stairs.
Most importantly, you should make it part of your routine. And you should do other things since you are enjoy. Hope this helps. Good luck and don't give up!