It is always important to stretch before exercising, whether it is a cardio workout or strength training. The reason for this is because your muscles need an opportunity to warm-up before beginning an intense or vigorous activity. Stretching also helps to prevent any potential injuries and/or prevent from pulling a muscle.

Stretching also helps with giving individuals the motivation to continue to exercise. It can be discouraging initially to know that you aren't as flexible as you thought. However, just after one week, you should be able to notice your flexibility improving.

Proper stretches should be held for 10-30 seconds and should be repeated 3 times to a point where you can feel the stretch, but it shouldn't be painful.

It is important to stretch the body parts you are planning on exercising. For example, if you only plan on running/walking for that day, it is necessary that you stretch your legs.  It is also important that you stretch the different muscle groups in the particular area.  Remember, you have several muscles involved in very simple movements.  It would be beneficial to be aware of these different muscle groups. 

On the other hand, it is always good to stretch all your limbs and torso before any exercise. In a way, it is working out your entire body even when you don't plan on strength training.

NOTE: Do not stretch for longer than 30 seconds because it may cause damage to the muscles.