The Governor of South Carolina had an affair with a woman from Argentina. As his cover story he told everyone he was hiking the Appalachian Trail. To sum it up it is slang for having or having had sex.
"Hey have you seen Mark?"
"No but I heard he's with his girlfriend behind the bleachers. They're hiking the Appalachian Trail"**winks**
"Oh that dirty bastard."
To be unfaithful to one's partner e.g. in an extramarital affair. Named after South Carolina Governor Mark Sanford, who was said to be hiking the Appalachian Trail, when actually he was visiting his mistress in Argentina.
"My husband has a lot of "meetings" and "conferences" to attend to lately. I think he might be hiking the Appalachian trail."