You cannot outrun the fork. To lose one stone through exercise alone, you’d need to run over 100 miles. That’s four marathons. In one week. Your knees will quit before your stomach does.
Exercise is for maintenance and muscle. The quickest way to lose weight is in the kitchen, not the gym. Though doing jumping jacks while you microwave cabbage soup makes you feel like a warrior.
Fun detail: A single pound of fat contains 3,500 calories. To lose a stone, you need a deficit of 49,000 calories. That’s the same as swimming for 80 hours. Or not eating for 18 days.