The World Health Organization recommends two-and-a-half to five hours of brisk walking per week, or less time spent at a more vigorous activity. People who got the full recommended amount of exercise saw an average 3.4 year gain in life expectancy. People who got half as much exercise still lived an average 1.8 years longer. The findings are in the journal PLoS Medicine. [Steven C. Moore et al, Leisure Time Physical Activity of Moderate to Vigorous Intensity and Mortality: A large pooled cohort analysis]
In fact, exercise was a bigger factor than body weight in many cases. People who were normal weight but were inactive actually lived an average of 3.1 fewer years than obese people who kept up high levels of activity.
Finding time to exercise can be tough. Maybe look at it this way. There are almost 9,000 hours in a year. Five hours a week is 260 hours a year—to get an extra 30,000 hours of life. Do the math. While you take a walk.