So did Conan get so cut from turning that wheel for so many years? 
Nelson Montana said:
After running a marathon, the average weight loss is four pounds. Three of which is water. Of the remaining one pound, at least half is muscle. That means it requires running 26 miles in a couple of hours in order to lose a few ounces of fat. How much fat do you think you're going to lose running 5 miles a week?
Aerobics are the big lie.
First off, running a marathon in a couple of hours would be a rather stunning feat if you have either excess bodyfat or muscle. By sheer numbers, on paper, yes, it would amount to at most a half pound of fat loss. However, what must be considered is how fat is utilized for energy in cardio activities repeated several times during a weekly period, in which case, there is different energy expenditure based on level of hydration, body demand vs. tissue nutrient availability, intensity, conditioning, external and still more internal factors, etc.
Aside from the fact that contrary to popular belief that cardio only burns fat while you are doing it, there are metabolic changes that accompany a sustained schedule cardio program. The efficiency that occurs is also a function of the adaptation that you are speaking of. One of the adaptations that occurs is the body's desire to speed the metabolism in attempt to prevent excess body mass from becoming an obstacle for future cardio work efforts. Additionally, the calories burned at the beginning of a cardio session are not the same types burned later in the same session. If they were, we would all be sweating within moments of starting cardio and there would generally be less of a decline in available energy. We're getting into a whole new ball game at this point as hormones and neurotransmitters are also key players in the game of determining available energy production pathways. So I will not delve into it too deeply as this is already going to be a long post.
What I believe you are not calculating is energy production and how it is affected by physiological mechanisms. Take for instance intensity & conditioning: both factors are relative in terms of how a given individual will respond to the stress of cardio. If we were all the same level of conditioning we would all have the same level of intensity. Human applied physics verifies that work and work duty are functions of effort, load and other factors. Greater load (person with more mass) is going to mirror greater effort for a given amount of work in comparison to a lesser load for the same work duty. In English, a heavier person walking up a long flight of stairs is going to have not only more exertion than a skinny person but greater load. This does not change nearly regardless of bodymass type. In the case of a skinny person and a bodybuilder walking up the long flight of stairs, the work effort is going to mirror the load capacity for both, but in quantifiable terms, it takes greater effort for the bodybuilder.
Adaptation can and does occur for many functions of the body. Adaptation is the shortcut of the body to do less work by way of lessening either load or work. The most successful way the body can do this is by way of bodymass reduction. Equal and opposing forces that cause a need for adapatation definitely apply in this case. If one is doing a ton of cardio, working out with progressive resistance training and eating according to good nutrition principles, then the body will have stimulus to be able to adapt to the workload of working out by maintaining muscle mass, as well as adapting to the cardio exercise by metabolically utilizing the tissue that will satisfy the adaptation need. In this case, it is fat. But let me underscore that there must be adequate nutritional principles in play here for this to be successful in terms of muscle mass maintenance and fat loss simultaneously.
If one ONLY works out with weights, unless one takes all measures possible to avoid adaptation (risking overtraining in some cases) then the body will attempt to adapt just the same albeit in a different mechanism. If this is not the case, then there would be no need for pro bodybuilders to ever need drugs for cutting...everyone would be ripped to the bone already just from eating right and lifting weights.
Yet I think we can all agree that if an overweight person started exercising by preparing for a triathlon and eating the right foods that their body would adapt by dropping weight. In my opinion, the big lie about cardio for many people may be in the readiness for the body to return to previous body fat set point should they cease activity. I won't argue with that.
But to say that cardio is a big lie in my opinion is far from the truth as many people have come to know. That it is just one part in the role of losing excess bodyfat I do agree.
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