As a work of non-fiction, A Walk in the Woods shouldn't be criticized for its frankly anti-climactic ending. In a way, this ending mimics the actual experience of walking the Appalachian Trail.
After reuniting, Bryson and Katz enter the Hundred Miles Wilderness, the most brutal section of the entire AT. Then Katz gets lost. Although the two men reunite the following day, both dudes are freaked out by how close Katz just came to a gruesome death. This also serves as a reminder to Bryson that they "weren't boys anymore," and that two old fogies like them shouldn't be wasting their time on the AT (2.20.33).
With that, they go back to their boring old lives. Bryson returns to New Hampshire to presumably write this book, while Katz returns to his job in Des Moines (and, thankfully, gets sober). Although we doubt that these two old friends will be hitting each other up on FaceTime any time soon, we're sure they will treasure their experiences together on the Appalachian Trail.