Walking is one of the best ways to workout aerobically and NOT injure yourself.  In order to make it count as an aerobic workout you will need to pick up your pace rather than just slowly stroll along.  Here are a few tips to make your walks really count toward burning more calories and to improving your aerobic condition.

1. ✦       Move your arms.  Keep your arms bent at the elbow and swing them quickly forward and backward, keeping them above your waist.  The faster you move your arms, the faster you will walk as your feet keep pace with your arms.  Keep your elbows close to your body and don't let your hands cross midline of your body as they swing forward and backward. Using of your arms while walking is one of the best ways to raise your heart rate and increase intensity during walking.
   

2. ✦      Focus on your feet.  Two things to keep in mind here to increase the power of your stride.  First, think about how your heel strikes the ground in front of you as you are walking. Really grab the ground with your heel. Second, as you roll forward from your heel in front of you to your toes as they move behind you, push off hard with your toes to propel yourself forward.  Grab the ground with your heel in front and then push off with your toes behind.  You will get the sensation of pulling and pushing yourself forward.  This is the difference between a stroll and a workout when you walk.
   

3. ✦       More steps, not longer steps.  Focus on increasing the number of steps you take rather than taking longer strides.  This will increase your speed and will also lessen the strain on your knees and hips.
   

4. ✦        Posture.  Lean forward slightly fron your ankles, not your waist to prevent back problems.  Try this:  Stand against a wall and lean forward, keeping your butt against the wall.  This is what you should NOT do. Feel the strain on your back?  To avoid this go back to standing agains the wall and this time lean forward from your ankles letting your rear end come with you away from the wall.  See how much better your back feels?  This is the way to move forward.
   

5. ✦       Core basics.  To create a solid strong core during your walk do the following:  Tighten your abs and gluts.  Lower your shoulders and pull them back.  Lengthen your neck, chin up.  Feel the difference. 
   

6. ✦       Breathe!!!  Oxygen helps.
   

My favorite book on fitness walking is by Therese Ikonian and is called (what else!) Fitness Walking published by Human Kinetics.