I ran a trail 100K last fall and my Watch survived the almost 13.5 hours. Granted, I was carrying my iPhone with me and wearing a chest strap HRM, so a lot of the work was offloaded. But still — I was impressed.
I’ve also run a road marathon with the watch without the phone.
The downside is if you’re using the Workouts app, then your data is silo’d. The upsides is that the data is local and encrypted.