Cardio ranks right up there with dinner at your in-laws' and a trip to the dentist. Like there's not a million other ways you'd rather spend your time! But you want to be fit and you want to be lean, so you do it.
This cycling of intensity gobbles up way more calories, both during the actual time you're exercising and for 24 hours afterward. That's important because the "afterburn"—called excess post-exercise oxygen consumption, or EPOC—is just as valuable in the overall equation of weight loss as the actual number of calories you burn during a given activity. Increasingly, studies have shown this type of workout trumps steady-state cardio in which you maintain a constant heart rate throughout the activity.
You want to have a baseline level of aerobic fitness before you start adding intervals. Anyone who has done at least a month of cardio work for at least 20 minutes three times per week has a minimum level of cardio fitness. At that point, you may gradually include a few intervals into your workout; over time. increase the frequency.