Having done the 24 workout mass blitz a few times I have to agree with Commander. The cardio certainly won’t help you with this program and might even detract from it. I know what you are talking about with wanting to work out on the off days though. When I first started this program I was coming off a 6 days a week split, so going to working out just 3 or 4 days left me with that itch to work out on off days. For the most part I got over it, learned to love the frequency of off days, but I did employ active recovery workouts from time to time.
"Heaven and earth shall pass away, but my words shall not pass away." - Matthew 24:35
"But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, meekness, temperance: against such there is no law." - Galations 5:22-23