Map My Walk is a free app which we use alot. Provided your phone is fully charged before leaving, Map My Walk is a great choice. We’ve used this app for many a long walk , some greater than 8 hours where all our other devices have run out of power. An added bonus are the “Voice Feedback” audible stats the app provides each mile. Pop your phone in the top pocket of your rucksack (so you can hear your feedback) and you can keep a track of your mileage without having to mess with your phone.
Check our running and walking posts for screenshots of our routes using this app.