The truth is you dont owe them any explaination, you could say something generic like "I no longer believe this job is in line with where I see my career heading". I wouldnt make something up though. If you want to tell them go ahead. Now that you have given your notice you cant be fired, they can ask you to leave but they still have to pay you for your notice weeks. I wouldnt worry about it, if you had a good relationship they will understand why you want to leave and thru-hike, if you didnt then likely they will forget you soon after you leave.