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"The problem is that aerobics or long duration steady state cardio turns your body into an efficient fat burning machine. On the surface this looks good.  Becoming more efficient means you use LESS. Think of a car. If your car was more efficient at burning gas, you'd be using less of it right? Not good.

The biggest problem with aerobic training is that you get better at it - again, the efficiency factor. With resistance training, as you get better you can add weight, more reps, etc. With endurance cardio as you become more and more efficient (better conditioned) the effort and energy and fuel cost to run for say 5 miles gets less and less. So if you're to improve you either have to run farther (do more cardio) or do it faster. So by this logic you're going to have to do more and more and more cardio over time to achieve the same effect that you used to get from less cardio. This assumes that as your conditioning improves you don't automatically work harder, which you should be anyway. That kinda sucks. This is why some people just do a ton. They have to. But you can only do so much. So then you have to do it faster - but there is an end point with aerobic training, that is, with sufficient intensity, it becomes anaerobic. So if you opted for running harder instead of longer (to achieve the same effect) you're going to hit a point where it's no longer aerobic in nature anyway.

Why not just do anaerobic work to begin with?

As with resistance training, with intense interval work - alternating periods of intense activity with short rest intervals - the above problems don't come into play. One, you're never doing it for a long time as it's not an aerobic task. Length of activity is not a goal (unless training for an endurance event). Two, fitness isn't an issue either because your goal is to always perform at or close to your maximum potential. So even as your conditioning and fitness improves, the adaptations aren't an issue because they enable you to perform at a higher level. If you got in better shape, and didn't pick up the pace and work harder, then sure, no more benefits after a certain point. But if you're working harder as your ability to work harder increases, you're following the overload principle. It's like weight training. If you can bench 100lbs and you always bench 100lbs, are you going to continually progress? Of course not.

So, where is the logic to doing an hour of steady state cardio twice per day? Over time you become efficient, which means this activity burns less and less energy over time. You'll have to do more to get the same caloric cost.

Simply not a wise use of time.

Score two points for interval training.

All that said, there is nothing wrong with steady state cardio. It too is a good tool to use when dieting. Too much HIIT while in a caloric deficit and coupled with a few intense lower body sessions per week will most likely have a negative impact on one, leg recovery and two, possibly lower body muscle loss. Extreme use of either HIIT (High Intensity Interval Training) OR steady state cardio is an unwise approach. Likely the most sane approach is a combination of both - say perhaps a couple intense interval sessions and a couple longer duration steady state sessions. Adjust accordingly from there."

Oxygen Website, 2007