Confusion about Cardio
There's a lot of confusion about cardio. Let me try to explain my point of view.
If you have lots of weight to lose you have two tools to use: working out and nutrition. SOMEHOW you need to empty the fat cells. It's done by burning off more energy every day than you need to maintain your weight.
Cardio training allows you to train and burn off a lot of energy. It's just an effective way to tap into the fat. If you do not expend a lot of energy via training it will most likely just take a lot longer.
There's a reason why the biggest loser contestants do 5 hours PLUS exercise daily to drop so much weight in short time. You must remember this is their job when they're on the show. You have a normal life with work duties, family, etc. so you need to lower your expectations and have patience.
Cardio training will not help you speed up your metabolism if you don't simultaneously build muscle. You need to ADD lean muscle since the less you weigh the less energy your body needs to maintain its weight.
Overweight people naturally burn more energy than someone petite. Someone who has 100 lbs excess body fat will burn more calories all day long than I do at 117 lbs for instance.
NOW, let's move over to when you're at a "normal" body weight or normal BMI.
If your goal is more muscle definition but you have not worked out long, you need to focus on weight training. Cardio training is good for a lot but it won't help you shape up. You're already in a healthy weight range so you must remember 'what do I want my body to do?'
Cardio training and weight training trains your body in different ways. You need to build muscle and allow time for this to happen. Excess cardio will slow down the process since it takes energy from what you need to build.
SOME people can do a lot of cardio and still maintain or increase muscle mass.
SOME people can't.
It's just the way it is.
Don't try to come around the catch 22 by doing HIIT every day. First of all you'll tax your central nervous system too much. Secondly, you won't be able to keep the intensity needed to qualify as a HIIT. If you want to do extra cardio for general energy expenditure, make sure you're doing it for the right reason... As in your diet is on point and you're working hard in the gym building muscle.
If you should do cardio or not: I like to check your strength. Someone who's strong compared to 'average' will most likely be able to handle more Cardio training without turning into a thin asparagus than someone who is a bit on the weak side.
You should be cautious of course when comparing yourself to others, but DENSE MUSCLE usually comes from being able to lift a lot for more reps than 5....
There's no short cuts, no cut to the chase answer and no clear path. We all have to go through trial and error, take notes and have patience.