Please don't take this the wrong way but their offer of offering you a job when you come back as long as you wait until 2018 to go is all about them and not about you. I have been in the corporate world for 20+ years and trust me, they want you to stay because they want you leaving to be on their terms. They want more time to prepare for your departure and for your replacement.
You were obviously ready and willing to quit and hike the AT. IMO, you will never regret going in 2017 but.... if you wait and something changes in 2017 to delay you or keep you from making the hike in 2018, you will have major regret.

In short, if you and your family are prepared and willing to make it happen this year... Do it... If you are good at your job, it will be there when you get back. That is, if you are even willing to continue in that field after the life changing hike on the AT.

Best wishes on your decision. I'm rooting for you!