Exercise is great, but you can't out-exercise a bad/high calorie diet. Running a mile on a treadmill (if I believe the computer) burns about 135 calories. A typical Big Mac meal is about 1,100 calories.
If I don't pay attention to what I eat, and am fairly inactive, I tend to eat about 50-100 calories more per day than I burn, amounting to a 5-10 lb/year gain.
That is enough of a gain that it really adds up over time, and once it has added up it is quite daunting to deal with, but as far as maintaining goes, it's very reasonable to just add a small amount of light exercise on a daily basis.
