When your goal is to lose fat – and lose it fast – your first temptation will probably be to ramp up the cardio. That's a mistake, at least in the early stages of your plan.
While we don't see it as much among hardcore iron-heads (it's more common among general fitness folks as well a competitive physique competitors) it's becoming more common.
Of course, cardio is not the devil. In reasonable amounts it won't eat away your muscles.
In fact, a few studies have found that people who dieted down while doing cardio (no lifting) lost less muscle and more fat than people who dieted down without doing any form of cardio or lifting. Of course, those who also lifted lost even less muscle (no loss or even a small gain).
But the message is that cardio, in acceptable amounts, can help you lose a bit more fat and won't lead to muscle loss.
The problem arises when people do too much "energy systems work" from the start – either too much per session, too many sessions, or going too hard right away. It's similar to what happens when you do too much lifting volume or cut calories too much. The body will eventually adapt and the amount of work you do no longer leads to significant fat loss.
I've worked with a lot of CrossFit participants ranging from normal folks to Games competitors. Even though they were fairly lean, they reached a point where they were no longer getting leaner despite a huge amount of daily physical activity.
I've seen some CrossFit athletes train 2-3 hours per day with fairly short rest periods (CrossFitters have a hard time resting even during heavy work). Yet for the time I've known them (2-3 years) their body compositions stayed the same, at least body-fat wise. In some cases, this happened even with strict eating.
The extreme case was one girl who was a freaky machine. She could bench press 225 pounds, clean and jerk 235 pounds, deadlift 425 pounds and could run a marathon any day.
She trained 2-3 hours a day with WODs and strength work five days a week and would go out running for another 2-3 hours twice a week. One day, as part of a charity event, she trained for 16 hours straight (1000 burpees, 1000 box jumps, 1000 thrusters, 1000 double-unders, 1000 wall-balls, a 1000-calorie row, and a 10 kilometer treadmill run).
She also did marathons, ultra marathons, and those crazy variations where you have to climb a mountain after your Ironman triathlon. Even with all of that work she was far from ripped. Her body comp never really changed.
So while you can't bypass the laws of thermodynamics, the body can adapt to excessive exercise like it does to excessive caloric restriction. Over time, the same amount of exercise has less and less of an effect on fat loss.
If you started out at 60-90 minutes of steady state cardio per day, 5-6 days a week on top of your lifting, where do you go when fat loss stalls? Do you up it to two hours a day on top of your lifting? What then? Add another hour?
First, it's not realistic unless you have no life. But more importantly, the cortisol production would be massive, leading to huge recovery issues, muscle loss, neurological fatigue, problems sleeping, etc. And ironically, chronic cortisol elevation slows down the fat loss process.
When it comes to the quantity of energy systems work, you want to use as little as needed to maintain the adequate fat loss rate: 2-3 pounds per week at first, maybe down to one pound per week once you're lean and are trying to get ripped. You might not even need any cardio at first.
Don't use the kitchen sink approach. It won't be sustainable. Use the least amount of work and restriction to achieve the proper rate of fat loss. Add work or reduce food intake as fat loss slows down.
As far as the hardcore energy systems work is concerned, understand that those high intensity intervals are a lot more demanding on your body than low intensity cardio, even if its shorter. The level of effort (intensity) is much higher and will raise cortisol and adrenaline more.
On top of that, if you don't have the cardiovascular capacities to do the high intensity intervals without feeling like you're about to die, the stress response is even higher.
A sedentary person, or even a serious lifter who's in poor cardiovascular shape, will do more harm than good by starting with intervals right out of the gate. It's smarter to gradually improve your cardiovascular capacities with steady state cardio and gradually work easier intervals in and later move to hard intervals.