First, your body will adopt in building muscle mass. At the same time, slower burn of fat will not be shown on scale as you lose fat and build muscles.
Once when you can sustain with certain muscle mass, your own Bodyweight, you will start to notice that you are able to do more work in shorter period of time.
This is when cardio adaptations occur.
Most importantly, have fun and stay healthy!
RE: HIIT and overweight population