A Sample Walking Program
During each week of this program, try to walk briskly at least 5 days per week. Always start with a 5 minute, slower-paced walk to warm up, and end with a 5 minute, slower-paced walk to cool down.

As you walk, check your pulse periodically to see whether you are moving within your target heart rate zone. See general-exercise/target-heart-rate-zone-table-t556.html on this site.

As you become more fit, try to walk within the upper range of your target zone. Gradually increase your brisk walking time from 30 to 60 minutes, most days of the week.

Source: http://www.nhlbi.nih.gov/health