First off what are your goals? Obviously if you are doing fasted cardio it is to lose fat. I would definitely do LISS type cardio (Low Intensity Steady-State) when doing fasted cardio. Interval type training is not required, from my experience. Keep your heartrate at 60-70% of max or ((220-age)0.6) to (220-age)0.7). Example (me) at 29yrs-old my range would be 114-133. I usually split the difference at 125 and hold it steady for 60 mintues every morning. Works great.
This will keep the cardio as an aerobic exercise, meaning you're body will use fat as energy. If your heartrate goes up, or you go for too long (~1 hour) your body will switch over to anerobic activity and chew up your LBM as vroom mentioned.
If your goal is to condition, then HIIT training is better. I don't recommend HIIT in the morning fasted, as you just won't get the full potential out of it.
Hope this helps.