I've been adding exercise to my diet and I think it's helping, although that may be psychological, and it also helps keep blood sugar levels stable (not spiking and dipping which can cause hunger pangs apparently).
I've read that it takes 3500 calories used to burn 1lb of fat (if we assume this is over and above diet/normal activity only).
Burning Calories - How to Walk for Weight Control
If you are 180lbs, and walk for 30 minutes a day for, say five days every week at a moderately quick pace, you are burning an extra 94 calories per mile, which at a speed of 3.5 mph for 30 minutes works out at 164 calories per day.
For five days, 5x164 = 820 calories burned by walking in a week.
Take this over one month: 4 weeks x 820 calories = 3280 calories.
That is 3280 extra calories burned per month due to the extra walking alone, which equates to just under 1lb of fat.
A lot of effort for not a huge gain, but the health benefits and the psychological benefits alone make it extremely worthwhile.
Walking Calories Calculator by Pace and Distance
Of course, this info is all from the interwebs, and I have no idea as to its accuracy