One way the American College of Sports Medicine defines your workout intensity is by your heart rate. Moderate exercise is defined as working at 64 percent to 76 percent of your maximum heart rate, and vigorous exercise is defined by the ASCM as 77 percent to 93 percent of your maximal heart rate.
To find your training heart rate zone use this link. You will need to enter your age and resting heart rate. The Karvonen formula is regarded as the gold standard for calculating exercise heart rate.
If you have been walking for a long time, it may be hard to elevate your heart rate during walks because your heart has become conditioned. Consider interval walk training to spice things up. (Check out this previous post for more information.) By increasing the intensity of the walk for three minutes (very fast, small steps with lots of arm swinging, OR light jogging) you should be able to increase your heart rate. To start, try three intervals of alternating fast pace for three minutes, then slow pace for three minutes.
Take a look at the American Heart Associations recommendations for physical activity in adults. Now that you know the difference between moderate and vigorous activity you are all set!