I’ve looked at all the settings I can think of for Google Maps and Google Now, and I can’t figure out what would cause Maps on your mobile to default to walking. The only thing I can think of doing is opening Google Maps on your device, then going to Navigator. Tap the little “curved arrow” icon on the top right of the search bar, then tap on the car icon to switch it to driving. Hopefully it will stick. You could also try going into your settings and clearing the cache and data on the Maps app.