If your goal is weight loss, and you have a choice between an extra hour of exercise, or eating one less Mars bar a week, skipping the Mars bar will be better for your weight loss.
Counting kcal per hour of sport like that doesn't make any sense. 300kcal is 1h of brisk walking (non-sedentary people do it as part of their daily life, and probably don't count it toward their physical activity). It's also 1h of weight lifting, which most people would probably consider intensive sport. OTOH it's only 30min of jogging, which is a utterly trivial amount of sport for any healthy person. Those are rough approximations obviously.
After clicking the "reply" button of this post, I'm leaving for training with my sports club. 30min brisk walk each way to get there, and 1.5h of sport that is estimmated to burn 500kcal/h. Burning calories is trivial for active people.
If your goal is weight loss you should probably skip a mars bar every now and then, but I think you're underestimating what's possible after a bit of training and fitness improvement.
Eating 3000 calories in one sitting is hard. Like, genuinely difficult, you're probably stuffing yourself uncomfortable and then in a food coma afterwards hard.
It reminds me of those silly TV shows with depressed 500lbs weirdos who just stuff their face all day. No-one normal is doing that. I mean christ it's only a bit less than half a kilo of straight mayonnaise.