Zone 3: Intense aerobic zone (70–80% of HRmax). This is the zone for endurance training. Your respiratory system, heart and blood vessels are growing stronger. You’re taking in more oxygen. In this zone, you’re burning about 50% carbohydrates and 50% fat, but you’re burning more calories overall. Because you’re working harder, this zone feels like jogging. You're sweating and working hard, but you can still talk to a friend.