You should consider all of your fitness and performance goals when determining what form of cardiovascular training is best for you. However, plank jacks are best thought of within the context of inserting some semblance of cardiovascular training into a resistance training session when time is short, or training your stabilizer muscles to maintain your body posture even during periods of intense fatigue. Certainly, they’re no substitute for a multi-hour running session if your ultimate objective is to complete a marathon.