I'm a 30 yr old woman. I weigh 170 lbs. I have 3 kids. I would like to know how many miles is good to walk without over doing it and how can I get into shape doing so.

The miles you walk aren't really limited, but you do need good shoes and a surface that isn't curved down on the sides. People who run on country roads tend to have all sorts of problems from one foot always landing lower than the other. That's why there are ovals to walk/run around at schools, or suburban side streets are good. A hike in the woods or along a rail trail is good too, because you are getting more of a workout.
Take it slow at first so you don't get blisters and give up.