I agree w/ the above.
Best time to do cardio for leaning and fat loss is in a glycogen depleted state. If the heart rate isn't TOO high, the body begins metabolozing fat as an energy source (since glycogen, and thus glucose is in short supply). And the more you do it, the more effecient the body becomes at it.
When are we maximally glycogen depleted?? In the AM fasting, and right after lifting.
I do 45 minutes of moderate heart rate cardio right after my 60 minute lifting sessions, and as soon as I'm done I drink a protein shake with simple carbs, to elevate insulin levels and replete the skeletal muscle glycogen stores, as well as shunt amino acids into the muscle cells.
Works great at leaning out and have seen no decrease in muscle mass vs no cardio.