I've been going for longer bike rides. A 100 km ride will more than double my calorie needs for the day. I eat a high calorie meal before I ride, I drink sugary beverages and eat chocolate bars (about 1 per hour) during the ride, and then I eat another big meal after I'm done. It's insane how much I need to eat when I'm actually getting exercise, I don't know what you're talking about.

Presuming one does not replace the burned calories, even walking 5km a day will burn the calorie equivalent of about 80 pounds of fat over the course of a year. It adds up. Conversely, not walking 5km a day adds up. After suffering a broken leg I spent a couple years largely sedentery in my late 30s, and I put on 25 pounds in that time. I snapped out of my stupor, started exercising again, and burned it off in short order.