Most of us get mentally tired by the end of the day. Our bodies were designed to be hunter gatherers. The physical activity involved in hunting the next meal would help digest the previous meal. As such our body is very efficient in optimizing energy use and stores extra nutrients for contingency in form of fats.
One thing you can do in your 9 to 5 day is to have a quick lunch and take a walk in the remaining lunch break. Make it a daily habit. Another thing you could do is either buy a treadmill or gym membership and get on the treadmill even at moderate speed for 40 minute sessions at least 4 times a week.
While doing the above, try to avoid eating between meals.