Hi Beverly,
Good for you for wanting to be sure you are working out to achieve results. Vigorous just means that you are challenging your body: that your intensity is increased throughout the workout. Essentially the amount of time you work out for is only one factor, and keep in mind that you can work at a very high intensity for a shorter time and have an even better workout.
You want to figure out your Maximal Heart Rate (MHR), which is a prediction of the maximum number of times your heart can beat in one minute to determine your intensity. Use the formula (220- your age), which would be 172 bpm (beats per minute). You should be aiming for about 75 percent of that number and try to maintain it for your workout.
Be sure that you are keeping your HR up, challenging yourself to work hard, and of course feeling it! You shouldn't be able to carry on an easy conversation if you are working out with intensity. So, bring along an ipod and concentrrate on getting in a great workout. Adding hills or intervals are a great way to up the intensity during your walk.
Keep up the great work!
Good Luck,
Paul