From those times when I was doing lots of boxing and sparring, I still do the shadow fight from time to time. It may look odd, but it is a real stress for the body, and the muscles got to work a lot. Also, it is a good way of maintaining active the neuron pathways for your movements. You can definitely consider it a cardio session if you do it right and intensively enough. If you had a punching bag it would be ideal, but since you are asking about the shadow fighting, I assume that you don't have a punching bag.