Without any special preparation my apple watch will last a full day of hiking while recording the GPS/heart rate/etc. (i.e. 5-7 hours hikes in the mountains.)
Cellular, on the other hand, can drain the watch relatively quickly. Mine lasts 2-3 hours when streaming music from the cloud and relaying to bluetooth headphones during a workout (i.e. when it can't piggy-back off the phone's cellular.)
There was speculation when the 8/X came out that they would have that feature. I would love it. Too many times, I have forgotten to charge my watch in the morning and only remember when I go to to run errands and see it in red. It’s not like you can just plug it into a lightning cable in the car and top off. I refuse to get a watch charger just for the car.
That's not possible (and probably ill advised as the watch will usually outlast the phone) - however it is possible to charge your devices from an iPad.
Cellular, on the other hand, can drain the watch relatively quickly. Mine lasts 2-3 hours when streaming music from the cloud and relaying to bluetooth headphones during a workout (i.e. when it can't piggy-back off the phone's cellular.)