I work 6 days a week. After school, I work 3 to 9. Weekends got me 10 to 9. My "exercise" is me running around, working fast, in a sauna.
On your day off, go for a jog, track your time and distance. Go for a mile. During the days you work, try doing something that will make you sweat. Jumping jacks, push-ups, lunges, planks, sprints, etc. But do them quickly one after the other so you actually burn calories. Do them prior to work, after, or during breaks. I do exercises during my breaks at work to keep myself awake and also burn calories.
Also Khorde has a good idea, although his is more proactive than passive.