Yep, and the diet part is the key because while you may burn 400 calories in an hour on the treadmill, you can eat a 400 calorie cookie in seconds. So I agree with the general idea that weight loss is 80% diet and 20% exercise, abs are made in the kitchen not the gym, etc.
Where the exercise comes in is keeping your metabolism from dropping off when you cut your calorie intake.
Indeed, for most people it's much easier to eat less calories than it is to burn a ton of them, but that's over complicating the issue and doesn't really need to be explicitly told to first-timers.
Let them figure that out on their own.
Once they have an appreciation for how many calories are jammed into that cookie, and how long they'll have to run on a treadmill to burn it off, they naturally won't be so interested in that cookie, which is great. Then show them how few calories are in an enormous garden salad.
Where the exercise comes in is keeping your metabolism from dropping off when you cut your calorie intake.