OP, if you ride hard enough to be breathing heavily, you are riding with an elevated heart rate. Ride at a pace that causes you to sweat (yeah, I know that doesn’t take much in Louisiana) and feel your heart beating. If you don’t have that feel, pedal harder and push to go faster over longer distances. No sensor required, and you’ll find your stamina improves quickly if you push yourself.
Of course, the gadgets and numbers are cool, but they are not required to get a workout.