TPM Reader SP is spot on:

Whatever Sanford has actually been doing, he's just made "hiking the Appalachian Trail" a euphemism for mysterious disappearances as a result of illicit activities.

Come home after a weekend blackout bender? Hey, honey, I was just hiking the Appalachian Trail.

Boss wants to know why you were out of the office for two hours while screwing a co-worker at a nearby hotel? Just a quick hike on the Appalachian Trail.

Not that we know what Sanford was really doing, but ...