Here's the deal with cardio -- it's great for burning calories, conditioning your cardiovascular system, and building strength and endurance. But it's not so great for building new muscle. In order for the physical process of muscle building to occur, a muscle has to be worked to exhaustion or failure within 60 - 90 seconds, which is about the time of a typical set of exercises while you're lifting weights. But as we all know from watching the seconds count down on the cardio machines
, we don't work our leg muscles to exhaustion within 60 to 90 seconds. Nope, most of us can go for 30 - 60 minutes at a time. So the physical process of muscle building (muscle exhaustion/failure, microscopic tears, healing stronger than before) just doesn't come into play with cardio.
My personal opinion is that both cardio and weights are essential for fat loss, along with good nutrition. Nutrition probably trumps exercise since we can certainly outeat anything we burn off at the gym, but in terms of exercise, we need both cardio and weights. Different types of exercise for different goals.
The first six months to a year that a sedentary person starts exercising are called the Golden Period because an untrained person tends to add muscle very quickly in the beginning. It's the ideal time to start a fat loss program because you really can add muscle while you're losing fat, even if you're in a calorie deficit.