A low intensity stroll at a 20 minute-mile pace or slower doesn't require any warm-up. Simply start walking, find your comfort zone, and continue. The moderate-intensity brisk walking will probably find that a 17- to 19-minute mile pace fell best for the first 1/4 to 1/2 mile before moving up to the full brisk pace. The high intensity aerobi walker will be mor fit and can easily handle a 15 minute mile pace from start for a warm up pace, gradually increasing speed after the first 1/2 mile.
Learn proper walking warm up techniques.