Yup. Good habits come first. Every 12 pack of pop is roughly a marathon, give or take.
The simple act of cutting pop for water saves a massive amount of calories, but can be really hard if you're not used to liquid not being strongly flavored.

Excercise, beyond being a bit self fulfilling ("I want to be in shape so it sucks less when I excercise"), does also increase your baseline metabolic rate once you get in the groove. Once you start moving your body becomes more free with burning energy when you're just sitting around, and you'll passively burn 3-6 cans of pop more than you would before.

But 100% to your main point. If you don't have your diet under control it doesn't matter how much you excercise, the medieval peasant that lives in your endocrine system will make it very hard to resist putting the calories right back in.