HIIT!

Tip of the Day: HIIT or High Intensity Interval Training has proven to be a much more effective fat burning and muscle sparing form of cardio than steady state, long duration cardio. If you are someone who loves long distance cardio, then by all means, continue to do what you love! Just know that if you are looking to burn fat and spare muscle, it just isn't going to happen over the long term with that type of cardio.

Long duration cardio eventually puts your body in a catabolic (muscle burning) state, because once your body has used all of your glycogen stores, it will then turn to burning your muscle for fuel. If you keep your cardio bouts to say, 20 minutes, you can effectively burn fat, increase endurance, and prevent your body from reaching the catabolic zone. 

A simple way to implement HIIT would be to do intervals of sprinting/walking on the treadmill, rather than just simply jogging at the same pace for a long period of time. If you prefer to do your cardio outdoors, you can do the same thing; sprint for 20-30 second intervals followed by 30 seconds to a minute of walking!