If your fat with no muscle mass then building fast twitch fibres to increase the amount of energy your body will use is key. Although,if you have muscle mass working it aerobically initially will increase muscular oxygen uptake efficiency before doing the fast short stuff will produce better results.

A thing called epoc (excess post-exercise oxygen consumption) accounts for the loss of weight following fast exercise but if the basic aerobic system isn't working efficiently you won't burn energy (fat) as well as you could.