Percentage of adult American women trying to lose weight at any given time: 35 - 40 percent 

There are many ways to perform an aerobic workout. The most important thing to remember about aerobics is that it is a "sustained" workout, ideally lasting at least
twenty minutes.

The intensity and speed you choose to work at is partially based on  the type of exercise you are performing. If you are a runner and your normal pace is 6 mph, your interval pace may not last longer then 8 to 20 seconds, depending on the speed you increase to during your interval. If you are in very good shape you may try running as fast as you can for eight to ten seconds.  If that does not sound like a long time, try it; you will be surprised to find how hard it is.  The key here is that you return to your normal pace of running after your interval pace, stay at your normal pace for a few minutes and repeat the same cycle. 

If you are performing the intervals properly it will affect your normal running time. For example, you normally run 6 MPH for 40 minutes, when you add in the intervals you may discover that you are tired after 25 minutes.

After a few weeks you should notice that your normal pacing time increases, and that you can handle the intensity better.

LONG SLOW DISTANCE
For a long time people believed that if you workout in a lower heart rate zone you would use more fat during your exercise.  Even though this is technically true, you would actually use just as much fat exercising at a higher intensity for a shorter time.  That is because overall even though percentage wise you use less fat at a higher intensity of exercise, you put out more overall calories. But what is more important than how much fat you use, is that when you exercise aerobically at a higher heart rate intensity, you get fitter!

Long slow distance (LSD) workouts do have their place. If you are a beginner or have not worked out for a long time it is a great way to build up your endurance levels. Also, if you are a sprinter or someone who does plenty of high intensity workouts like spinning classes this is a nice way to change your routine. 

One final note on Long Slow Distance exercises, this type of workout is designed to go at least 90 minutes non-stop. So make sure you have plenty of fluids and even some Gatorade on hand is a good idea for this type of workout.

BLOOD POOLING
You're working out at an intense pace, you're tired so you decide to just stop; wrong choice!

When you stop moving your legs in the middle of your workout, you create an effect called "blood pooling". The effect is caused when your heart is pumping hard pushing the blood down towards your legs and your legs are pushing the blood back up toward your brain. By suddenly stopping, your heart is still beating fast pushing the blood in your body downwards but your legs are not returning the blood back up at the same pace; this causes more blood to be below the waist and less blood gets to the brain. This can cause you to become light headed or even pass out.  

So take a few minutes at the end of your workout to slow down, walk in place if you have to or if you need to sit down, pump your heels up and down slowly until you feel recovered from your workout.

JUST DO IT
The most important thing about doing cardiovascular workouts is doing it.  Over time you should see improvements in everyday actives like walking up stairs without being winded and your overall endurance improving.

Get yourself a heart rate monitor to help pace yourself as you go. Use the formulas (listed on the Measuring Your Heart Rate page) to figure out your target zones that you should be working out in.

Most importantly listen to your body! If you become red in the face while exercising, then you need to slow down.  Make sure that you have a five minute warm up and a five minute cool down time built into your routine.