It occurs to me while reading your post that perhaps your "6" is more like a 8 or 9 in reality. Maybe you're starting off too hard and fast!? Remember the analogies Phillips uses? Level 5 is like carrying a bag of groceries" (or whatever it was)..... I don't have the book with me, but perhaps if you reviewed that portion of the book and re-evaluated your levels of intensity? Remember, you don't want to hit your 10 until the very end!

And there's nothing wrong with taking it easy at first.... TrainerDan suggests taking 2 weeks to get the feel of BFL before going at it full throttle.

Whenever I am disappointed with my workout effort I always remind myself (in a huff), "Well, it's better than sittin' on the couch"

You're doing just fine