I don't think trying to passively exercise while working is the right approach - I take 2-3 15 minute walking breaks a day, but I also do 30 minutes to an hour of cardio before or after work every day. I use a bike trainer and rowing machine, but this could also be running, HIIT, roller blading, whatever. IME getting your heartrate into the aerobic zone for 30 minutes a day is way more effective at maintaining fitness than having it slightly elevated all the time.