Well, you must permanently change something, at least. Which something is easier might vary from person to person. And it’s pretty easy to fit exercise into your daily routine if you don’t schedule it as “exercise”: Biking to work instead of driving, for instance, or taking the stairs instead of the elevator.
For a lot of us exercise goes the other way psychologically too. I do not enjoy exercise, but I do it every day anyway because of the options of a. getting fat b. giving up sweets or c. exercising, c is the least unpalatable. But I don’t exercise for very long most days because I don’t have time to. When I do get in a long work out, and spend a couple of hours burning several hundred calories (up to 1200 if DDR’s calorie count is to be believed) you’d think that I’d feel even more justified than usual eating something yummy. But I don’t. Those calories were damn hard work burning, so I find that I’m better able to resist temptation once I’m sitting there sore and tired, than I am when I do my usual work out that only burns a hundred or two hundred calories.