People have put pedometer's on their shoes and some say the average between 7-9 miles.
The key is to plan and don't do to much back tracking from one area to another.
Our first time there, we missed Adventure Land at the MK.
When we went back we started out in Frontierland, then went to Adventure Lane, then to tomorrow land. We didn't go to Mickey's Toon land and missed snowwhite, pooh, bears jamboree. We finished up with the Haunted Mansion and the Riverboat and waited for the nite time Spectro Magic parade. Frontierland is a great place to watch the parade at nite-hardly anyone there, and this was Easter week.
Good luck.