It's not that you will need 18 days of battery life, it's simply knowing that you have it there. If you're going off for 3-4 days on a hike, there's a possibility your trip is getting extended by bad weather, wild animals, and whatnot. Knowing that's a possibility you always pack extra gear, and the idea of your main map on the trail dying is not an option.
That fits my use. I go on 1- to 4-hour walks pretty regularly and the existing Apple Watch is already helpful for rough distance/time/calorie tracking. If the Apple Watch didn't exist, I'd probably have a Garmin, but I'm pretty happy with the "good enough" tracking features on it. I like that the Ultra shows that they care about this market.
The extra battery life of the Ultra will be nice as will the new apps. Garmin still makes more serious "adventure" watches but for really hard-core mountaineering, I'm not sure to what degree people actually depend on watches that still need periodic charging.