CARDIO workout

I posted this a little while ago but know1 replied to it. I am new to this idea of cardio in the am. I have currently switched my cardio from doing intense runs to the 65% mhr for 1 hour. You guys here say that it is much better to do it on an emtpy stomach in the am and then eat 45 mins prior to the cardio. Wouldnt this be extremely catabolic because your body has fasted for the past 10 hours. The glycogen storage are depleted and now your requiring energy to perform the cardio...wouldnt it go to your muscles for protein..Im not saying that it is wrong its just how i understand it..Can some1 explain the process to me..thank you