Racewalking is like power walking on steroids. In fact, elite racewalkers can move as fast as most people run. The exaggerated arm-pumping and hip swaying actions are a bizarre sight to behold, but racewalking is a slightly lower impact alternative to running that gets your heart rate higher than walking does. This video by Ian Whatley gives you helpful basic techniques without overwhelming you with all of the nuances. And it does get highly nuanced: Unlike with running, there is very strict proper racewalking form. One foot must remain in contact with the ground at all times and the other supporting leg must remain straight through most of the movement until your body passes over it. Because of these restrictions, your stride length is generally shorter, your abs and legs work really hard and vigorously pumping your arms helps your rhythm, not to mention burns more kilojoules.
Additionally, Racewalk.com and eRaceWalk.com are more helpful resources, breaking down what you need to know to start racewalking and even showing the mechanical differences between running and racewalking. In the Olympics, they typically racewalk for 20 or 50km, but you obviously don't have to racewalk that distance to get cardio benefits.
Racewalking - Basics [Ian Whatley]