"Feel the burn"?
This is not the way strength training is meant to feel - it is meant to feel different from cardio.
Strength training is supposed to be done in a slow and controlled manner, and typically you will just feel you cannot do another rep with the correct form. Doing crunches for as long and fast as you can in the way you might do cardio, is NOT the way you are meant to be doing them.
I agree with the suggestion of planks - they work a much broader range of core muscles than crunches, and most people find them easier on the back. www.sparkpeople.com/resource/exercises.asp