To create energy, your body first uses the the sugar in your bloodstream and your carbohydrate storage. Then it starts to break down your body fat to replenish the glycogen levels your exercise drained, which starts around 45 minutes into your workout depending on how hard you're working and how 'carby' your diet is. Corey says the goal is to burn through your carbohydrate supply quickly so that your metabolism is forced to work over-time – possibly four to eight hours post-workout – to replenish your body's glycogen levels, burning extra calories the entire time [called the after-burn effect].
Again this all depends on your specific body composition and diet – but most people will burn through their carbohydrate supply in much less than 45 minutes doing high-intensity cardio. But HIIT is not for everyone, and there's nothing wrong with that. If you're going to workout at a lower intensity (around a 5, on a scale of one to 10), Corey just recommends exercising for at least 45 minutes to tap into your fat storage and increase the after-burn effect.
Here are 25 workout moves you can incorporate into an exercise routine that goes at your pace.