Walking is a very safe sport because it rarely causes injuries. Running causes injuries frequently because you take both feet off the ground at the same time and land with a tremendous force that can tear muscles and shatter bones. On the other hand, when you walk, you always keep at least one foot on the ground and land with minimal foot strike force.
If you want to walk to become fit, you have to move fast. You should exercise vigorously enough to increase your heart rate at least 20 beats a minute higher than when you rest. That means you will be breathing harder and probably perspiring. There are two ways to increase your walking speed. You can take longer steps or you can move your feet at a faster rate. To lengthen your stride, swivel your hips so you reach out further forward with your feet. This will cause you to twist your body from side to side, which will tend to make you point your toes inward when your feet touch the ground. When you point in you lose distance. Try to point your toes forward with each step.
To move your feet at a faster rate, you have to move your arms faster. Every time you move one leg forward, you pull the arm on the same side back and the arm on the other side moves forward. Your legs will only move as fast as you can move your arms. Bend your elbows so you can move your arms faster. The fulcrum of your arm-swing is at your shoulder. The straighter you keep your elbows, the longer it takes your arms to swing forward and back. Bending your elbows shortens the swing and helps you speed up your pace.