Yes indeed, intervals again. Remember that the key to a successfully maintained workout is to approach that first round with a heart rate and pace you're prepared to maintain. At the end of that last calorie you should hopefully look at the clock, and think to yourself, "that hurt but I can do that again." It shouldn't be comfortable, but it should be repeatable. So see how close you can make each round to each other and let's have some fun with it, YEAH!