Sue22 gives a good suggestion on how to get an average, but it depends on so much, that unless you walk the same route each time the converting from steps to miles will be somewhat unreliable. Counting steps is great for motivation but does not tell you much else.
You step length changes depending on speed, crowded or open space, straight walking or moving around people, surface, shoes, weather, light, tirednes and in-/decline. It even depends on if you have company while walking.