If you want to burn the fat and lose more weight then your should do high intensity cardio workouts.
If you just want to maintain your fitness level then you can do only low intensity cardio.

Heart rate will depend on your age. To burn fat and spare muscle I try to keep my heart rate between 135 and 150 bpm for 30 min at least 3 days/week. Fasted cardio would be best or do your cardio AFTER you lift weights. You should have used up most of your glycogen stores during your lifting session so you will now use fat as the energy source instead of your glycogen reserves. Now if doing HIIT that is a whole different animal, but I am assuming you are talking about steady-state.