Agreed, you cannot outrun a bad diet. If you want to lose weight by shedding the excess fat you've accumulated over the years you have to address your diet first and foremost.
What you need exercise is for your overall long-term health and wellbeing, to "make you more resilient and prevent or delay many of the diseases that will make your life miserable so you can enjoy a higher quality of life much longer."
CICO, yes, 3500 calories = 500g of bodyweight so if you maintain a daily 500 calorie deficit for 7 days you should lose 500g at the end of the week
but even after you hit your goal weight you'll have to keep counting calories of every meal you eat to maintain that weight loss - doesn't seem very sustainable long-term