In general terms, only apps with an Apple Watch module will work on the watch. I also have MotionX-GPS on my watch but it relies on the iPhone so not my first option as I would like to keep phone turned off during the day so I can be fully 'disconnected'.
There are numerous hiking/running/walking apps available for the iWatch but what I like about this one (not fully tested so still hypothetical) is that it does not rely on a phone to work. You can download maps as needed for offline use and since the iWatch 4 has built in GPS it is fully independent. The $64,000 question is what you asked: How long will the battery last? Since I wouldn't be recording my track I imagine one could simply turn off the app when not needed but when needed simply turn it on, see where you are and where you need to go and then turn it off again, thus saving lots of watch battery power. If one wanted to record daily distances, altitudes, etc you would need to keep watch on but not sure how long charge will last in that case. I will start testing to see but so far on my short outings I've been impressed with maps and accuracy. I will also load a GPX track to test however on CF and CP this really isn't needed. For less well marked routes this would likely be more important.