Whether you are walking for fitness or weight loss, you should walk fast enough to achieve a fat-burning walking pace. Walking at a brisk pace is a moderate-intensity cardio exercise. The speed of a brisk pace is different for each person as it depends on your heart rate, age, and level of fitness.
A brisk walking pace can be from 13 to 20 minutes per mile, or from 3.0 mph to 4.5 mph. At this pace, you should be breathing noticeably but able to speak in full sentences. If your walking pace is 20 minutes per mile it may be either fast enough to be moderate-intensity exercise or too slow. If you are more fit, it might be only be light-intensity exercise. You can only determine this by understanding what the moderate-intensity zone feels like and what heart rate will put you into that zone.
Excerpted from Very Well