Like everyone has said, definitely take a week off and "play" at your training. Do it like Eugene said, about 2x most people.
As to the Zone, you and I are almost the same size (5'11" and 157-160 here). I stay at 15-16 blocks.
I felt as you do when I first started the Zone.
Make sure you are eating enough, especially your fruits and veggies. It can be tough to eat your 4 blocks of carbs.
3 cups of broccoli, for example, is 1 block. So a 1 pound bag of frozen broccoli contains around 20 grams of carbs. You eat that and still need another 16 or so grams to get to 4 blocks (an orange, an apple, a pear, a couple of peaches, more veggies ...).
Once I began to closely monitor my carb intake, I quickly felt much better. I have also added fish oil supplements with excellent results.