Eat a balanced diet that contains a good amount of healthy carbohydrates. Whole grains, fruits and vegetables provide a steady source of energy that will allow you to work out for a longer period of time without feeling tired or weak.
Always listen to your body when increasing the length or intensity of a workout. Discomfort is common, but if the pain becomes unbearable, you need to stop or adjust the routine to prevent injuries. Learn to listen to the limits of your own body.