With various types of exercises, class formats, and philosophies we have so many options on how hard we decide to work out. If you exercise at the top of your maximum heart rate range it's important not to push beyond your limits, but still keep your heart rate up. Working-out 2-3 times per week at your heart rate maximum is all it takes to increase endurance.
Doing my homework the other night, I attended a core boot camp class. Keep in mind every boot camp class is a bit different - it was totally my hard-core style and met the needs of every participant.
What's your preferred intensity - do you push it to the limits?