Speeds vary among individuals, but brisk walking usually requires you to move at least 5 km/h, ie you should walk 1 km within 12 minutes. You should experience a slight elevation of your heart rate, heavier breathing and you should start sweating a little after a while.
It has been suggested that there is a relationship between the speed of walking and mortality, and that the best results are obtained with a speed of more than 4 km/hr.
Walking fast vs big stride
Over-striding is a poor technique to increase speed and it has a potential to lead to injury in the long term unless you do it the correct way. The best way to brisk walking is still walking fast with short strides.
Video by HPB: The Correct Way to Brisk Walking for Better Health