Training with a heart rate monitor is a good idea, yet it is at best 13% accurate because of individual variations in heart rate and metabolic marker points.
Your anaerobic or lactate threshold is the most important factor in determining where you can burn the most calories in extended exercise, which means better fitness and weight loss.
Gas exchange testing is the most precise and time-efficient means to determine threshold points and the resulting individual training zones.
The test itself, which is MAXIMAL, is only 9-14 minutes on a bicycle ergometer something anybody with any level of fitness can complete. If you are a competitive runner or triathlete, you can test on a treadmill.
Use your valuable exercise time with a program designed specifically around your abilities.