Trying to get in shape. the muscles in my legs are so weak after a five min. cardio workout. i can barely stand after. why?

Stick with it!: Anytime you start a workout regimen, especially if you were out-of-shape before, there is always a build up phase. If you stick with your workout program you will notice that your endurance will improve with time. Try mixing up your workout regimen and not doing the same thing every time. Make sure to stay well hydrated and eat well balanced meals. Try eating a banana before working out.