thats true. I know I just came back from my first big workout (lower body, and cardio) and I swear I have like no energy and I am gassed! lol I can surely tell the difference.
You may want to supplement in some monster (type) drinks pre-workout.... I know a lot of people swear by them... or similar to help get that boost during cardio... also blow a fan on you. You'd be surprised how much more cardio you can do with your body temperature slightly cooled.