Cardio in the morning then lifting later in the day is fine. What's a waste of time IMO is doing them at the same time or doing cardio later in the day after you lifted. In order to build ANY muscle, you have to take in more calories than you expend in your workout. To burn fat, you have to use up more calories throughout the day than you take in. Personally, I seem to have MUCH faster results when I split the days, especially since you need to eat a lot more when you're trying to build muscle than when your aim is to burn fat.