Damn..what about you calorie intake? and did you feel like you lost muscle?
heartrate between 40-60%??? at 40% you are hardly burning anything in fact i bet most guys couldn't even get get on a cardiomachine and keep heartrate that low. even at 60% you are burning very little energy but a high percentage comes from fat. that however does not mean that its the best thing for fatloss.
you never burn off just fat. even at low intensities you burn a mix of carb and fat about 50/50 for a normal person and a bigger percentage of fat the better you are trained at burning fats.
and burning glycogen is not a bad thing. you simply need to put your body in an energy deficit and when you do that you will lose weight.
if you do 20 min at a very low intesity you will burn so very little energy that the impact will be negliable. try doing 45 min for a start.
Damn..what about you calorie intake? and did you feel like you lost muscle?