An app, such as the one I used earlier today for checking how far I walked, uses data (free of charge) from GPS satellites. It doesn't connect to the mobile phone network and therefore needs to use no chargeable data at all:
https://www.mapmywalk.com
It's a freebie. You have to register for an account online but it's a simple, free process and you don't get bombarded with emails or ads through doing so. According to many review sites, and the technical press, it's by far the most popular app for tracking and measuring walking activity.