Suppose that i need 1000 kcal a day as BMR and i am a sendentary person. I eat at max 1000 kcal a day. If i start brisk walking for 500 kcal a day, that would mean, considering a 70% of energy source during brisk walking coming from fat burning, that i burn 350 kcal from fat. The caloric deficit will be compensated by fat burning again, so, let's consider another 350 kcal from fat. That would sum up to 700 kcal from fat during a time span of 24h. After ten days, i should lost one kilogram of fat (1kg = 7000 kcal), three in a month. Is this calculation in principle correct?
The caloric deficit will be compensated by fat burning again, so, let's consider another 350 kcal from fat.
This is not what happens. Your caloric deficit was generated by the brisk walk. If you were to eat an additional 500 calories, then the fat/energy burned during the brisk walk will be replenished. At the end of the day your calories burned / fat lost will be net 0.
Technically, in your scenario, you are in a 500 calories deficit by the end of your walk, but you have to eat eventually. If you waited the next day and ate 1500 calories throughout the day while only burning only 1000 because you didn't do the walk, then you are back to a net 0 deficit.
Calories burned is an accumulation of work performed throughout the day. The cause of that work doesn't matter. That accumulation is called your Total Daily Energy Expenditure (TDEE). Walking, running, standing, digesting, breathing, heart beating, thinking and so on is all included in this.
Calories consumed comes from all the food you eat.
A calorie deficit occurs when the TDEE is higher than the calories consumed. How much of that is fat, glycogen, or lean mass is near impossible to accurately calculate and frankly not worth it. It will always be some portion of those three so being in a caloric deficit over time will ensure fat loss.