Keeping your heart rate in the zone I believe was done to keep cardio safer for the masses. Another big selling point was in the zone your burning more fat for fuel instead of sugar. It was thought this is a magic property for fat loss. The real key is total caloric cost. Meaning the harder you go the better it is for both fat loss and improving your cardio endurance. I never consider the heart rate when training. Perceived level of exertion is good enough for me. I time myself if I'm outdoors and when on a treadmill the measurements are built into the machine. Trying to keep myself at below a certain heart rate is a waste of time.
