oh yeah

So I did 30 minutes of cardio and then went over to lift some weights pretty fast. It may not have been anything, but I got lightheaded for about 5 minutes.
All the reading I've been doing does recommend light cardio and light lifting - A juice fast isn't supposed to be done while trying to have intense and high impact exercise, because it doesn't give you the stuff your body needs to recover... if you work out intensely, then use juice as a daily supplement to your meals, or an easy and healthy way to substitute a possibly unhealthy breakfast/lunch/dinner to your daily routine.
Also, you can use supplements and powders (L-Glutamine, creatine, whey, etc etc.) in your juices. I'm not taking that approach because this is for weight loss and also a reboot of my system. If you're interested in trying this, here's a great link with several different plans that you can try.

Juice Plans

Again, I don't know if people are reading this, but I'm trying to give a normal person's detail of this kind of fast. So hopefully people are benefiting from this...