You gained weight because you ate too many carbs, which raised your blood sugar, which increased the levels of insulin in your blood, which forces your fat storage mechanism to store fat instead of burning it. Exercise and calorie deficits are a convenient way to explain weight loss ideas, but they are totally wrong, as your example here clearly proves. I eat WAY more than my daily allotment, and I still lose weight. How? Not because I exercise, and clearly not because of a calorie 'deficit', but because I keep my insulin levels low, by not eating excessive sugar (aka carbs). For me, that daily number is around 80-100g for weight loss, and 100-140 or so to maintain weight. I am a fit man, and 196cm (6'5") so your numbers may vary, but that isn't a bad place to start. If you can stick to well under a 100g a day, you WILL lose weight, and you don't have to take a step, let alone 12,000, to do it. By the way, I am not saying don't exercise here, exercise is great, just not as a weight loss tool. Forget all conventional wisdom and excuses, they all based on a flawed premise.