You don’t need to be in the gym for hours on end to achieve your goals, worry less about working single muscle groups and concentrate on compound moves that work multiple muscle groups in one movement. e.g Burpees
While doing large compound exercises multiple muscles require energy at the same time. In order for those muscles to receive an energy supply, the heart rate will increase because energy is delivered via the blood stream, which in turn increases the amount of energy burned, making your workout a lot more effective.
Remember working more muscles at the same time, SAVES time!!