As well as mounting your phone safely on the dash with a proper holder, you'll also want a car charger, as Google Maps running constantly will sap your battery very quickly.
Any app which constantly uses GPS (such as MapMyWalk, as well as motoring satnav apps) will use a lot of battery resources. So, wherever possible, it's best to have a USB charging lead plugged into a phone that's being used by such an app.
fourteen, Google Maps uses the 'free' GPS service, not data so very little data is used.
It is possible to download the map and route over wifi before you set off.