I have been lifting weights for about three weeks now. I feel good about every workout. My question is when and how much cardio should I do. The first week I ran for 20 minutes and this the stairmaster for 10. This was way hard on my knees. Now I have been walking at a very brisk pace on the treadmill at the steepest incline for 40-45 minutes. I really want to lose weight. I do this everytime I workout. (5 days a week) Any advice will be appreciated.