Hiking may not be a job, but thru hiking is. It's a short commute to work, the hours are flexible and the work environment is (usually) great, but it's something you pretty much have to do every single day. For months and months. Many, if not most, of those who embark on this journey have never done anything like this before and have no prior experience.
I don't know how one mentally prepares for this. You end up either loving it or having the blind determination to finish no matter what. Everyone else goes home before the end.