I put my phone on the stem.
In a simple AliExpress splash/shock-proof bag( an iPhone 6 )
Getting the 20% battery warning after about 4:30h( wifi, bluetooth, and 3G and all useless apps turned off).
Sometimes I use a Bluetooth heart rate monitor to check how I fare on my weekly rides(2-3h) too.
Adding a 3500mAh Verbatim power pack that for 65g/25€, can more than double the time when needed.
The app I use for nav is ViewRanger, it is free, and allows an offline load of the maps.
It gets better at each release.
Then Strava+HRM if I need just to record my ride. It is extremely battery efficient.
Recently I was lost and used Google maps in bike mode, ending at 80% battery after an hour(this was not the case before).
When a better(design, gps, glonass,...) rugged Android comes to market.
I'll probably switch and get rid of the bag, as the touch through the plastic membrane is not optimal(my only complaint of this setup).
I think mobiles and software are good enough today for nav and ride recording, than having an additional device(I take my phone with me in case of emergency anyway).