Marathons and 20 mile trail races are not remotely long. A thru hiking day is usually about as long as the available daylight hours, so 16 hours in summer. A 100K race typically falls in the 10 to 20 hour range, depending on the terrain and the personal fitness. A 100 miler is usually over 30 hours for sub elites. An entry level Coros watch can handle all of these, there is no need to bring up premium priced Garmins, and certainly not much justification for the Apple Watch Ultra.

Few people do those types of races and The ultra 4 can handle the times you are describing.

From apple website: Apple Watch Ultra Workout

Up to 18 hours workout tracking Up to 25 hours Extended Workout tracking Up to 45 hours Max Extended Workout tracking

My experience with the none ultra is that the battery life estimates are spot on.