If it's a short walk, you can do it several times a day, trying to shave a few seconds off your previous best time. You can count the steps per minute to see how fast you are walking. Once you have a route to walk, measure the distance. When you know the time and distance, you'll be able to calculate your speed in miles per hour. These numbers are for a 120 pound person.
Heavier persons burn more calories.
Another way to keep your speed is to listen to a tape that has a steady number of beats per second. A company called SportsMusic has a series of tapes at different exercise levels for walking, biking, treadmills and other machines, with tapes and CDs ranging from dixieland to pop, showtunes to classical. Walk on even surfaces, but do some stepping up and down on curbs and add some hills to vary your workout. Walking has the least possibility of injury of any of the popular exercises. The aerobic benefit from other exercises compared to walking at 3 miles per hour is shown below.