How to build up endurance for INSANITY workout?

Lift light weights many times. Also, do breathing exercises to eliminate asthma. I used to have asthma to. Never accept it though. I used to have asthma which I overcame. I can now hold my breath for nearly five minutes and run a mile in 4:38. Best of luck, and I hope you do very well on your weight loss journey, and get the results you want.