whats your fitness level? you might just need to work up to it. modify when needed, pause, get some water, maybe have a banana before or during (the potassium and sugar helps keep you going). ater 10 days try level 2, if it still feels hard, just keep doing level 1 till you feel ready.
Jillian did say on her show one time the workout is supposed to shred off 500 calories in 20 minutes... so it shouldn't be easy.