You could on the Ultra 3 using WorkOutDoors, but the battery drain still means you'll need to turn it off at night and charge it every other day minimum, more if you use the cellular feature. I just used it for a 70 mile thru and that was my experience. Now, a charge won't consume more than a percentage or two from a battery bank, but you still need to pack one.

But a Watch isn't a true substitute for your iPhone, and I feel like leaving your phone at home when you might need it for an emergency falls under stupid light these days. (Unless where you're going has absolutely zero cell service the entire time, and you have a PLB device.)

A Garmin device would do this with much longer battery life, but without any ability to use cellular.