To become fit you need to exercise vigorously enough to increase your heart rate by at least 20 beats a minute. Walking slowly doesn't make you fit.
There are two ways to walk faster: take longer steps or move your feet faster. To lengthen your stride, twist your hips from side to side and reach forward with your feet. Pointing your feet forward after your heel strikes the ground helps you gain a few inches.
It's easier for most people to increase their speed than to lengthen their stride. If you move your arms faster, your feet will move faster also. Every time one leg moves forward, the arm on the same side moves back and the arm on the other side moves forward. For every step forward, there is an equal number of arm movements forward. It takes more time for your arms to swing further with straight elbows, so keep your elbows bent.
You may find that it helps to count out a cadence to yourself as you walk, 1-2-3-4, 1-2-3-4 — as if you were marching — or hum a tune, or play some peppy music into headphones. (Do not wear headphones if you are walking where there is traffic.)