With the fitness industry being drowned in so many myths and fallacies about training, people can be forgiven for having the wrong idea about what works and what doesn't!
Fat loss or 'Body Composition Training' has to be the number one goal of at least 90% of gym users, yet on any day of the week in any gym in the world, the lunch hour will see the majority of people religiously doing their '20 minutes on the treadmill and 10 minutes on the x-trainer' expecting to wake up with the toned, slim physique they dream of..
Our bodies are dramatically complex and what we are able to achieve is partially limited by our genetic potential, (only partially! read 'biology o belief' by Bruce Lipton) but more so, by our willingness to ask the right questions of our bodies and provide them with the correct environment to change!
My point is that, your body will adapt specifically to what you ask it to do. No more and no less! The fact that an individual may 'want' to drop 20lbs of body fat and so asks the body to perform 30 minutes of low intensity, steady state cardio 2-3 times x week while making no nutritional changes and gets absolutely no where, should come as no surprise. The only adaption that will take place is the body getting more energy efficient at completing that same 30 minutes of cardiovascular exercise! As energy conservation and efficiency is the bodies no.1 survival priority, it will adapt by improving at the completion of a given task. Doing the same thing day in day out without a thought to progression and expecting change is ridiculous!
Steady state cardio is particularly poor for fat loss/body composition change as it is carried out at too low a heart rate to create enough of a metabolic disturbance. If you were to work much harder you couldn't go for as long. It will also, do nothing to maintain or promote lean body mass!! (muscle). Unless you run further or faster each time you get on that treadmill, you are simply asking the system to get better at the same task and ultimately use less and less energy each time to complete it. The combination of traditional C.V with poor nutrition and no activity that promotes lean body mass is the reason that you could get fatter through exercise and the real killer is, that's exactly what you asked for!