Just a quick reply before I begin my running and walking exercises for the day...
My GPS watch is my single most important piece of walking gear. I currently have a Garmin Fenix 6 and before that I had a Garmin Vivoactive and a Vivoactive 3.
Before buying my first GPX watch, I tried to use my phone as has been described. The battery would run down too quickly so I carried a USB battery which I would plug into the phone and carry both while the phone charged.
The Vivoactive watch was better than the phone, but its battery would only last about 7 hours on a walk, so I bought a short cable to charge it on the move (while still on my wrist!). It worked, but it wasn't convenient.
The Vivoactive 3 was much better in many respects and the battery would last a little over 8 hours. I had some niggles with it though (eg. I didn't like the touchscreen) so for Christmas my wife upgraded me to the Fenix 6.
The Garmin Fenix 6 is an incredible watch, but I'll only mention the battery life for now... We did a walk the other day that lasted nearly 10 hours and the watch still had 75% battery remaining at the end. And that's in default mode - there are other modes that allow it to run for weeks without charging!
That's one difference between a good GPS watch and a typical smartphone. More will follow!