John Andersen provided an updated route description:

Rick Martin proposed this route:

Milt Splik proposed this section as an FKT route:

From the Scott Farm AT Center near Carlisle, PA to the Doyle Hotel in Duncannon, PA is about 15 miles. So, a round trip is in the neighborhood of 30 miles. I've done this hike several times, and will now establish it as a section for tracking the FKT. 

I think the proper way to do this section is to eat a burger and drink a beer at the Doyle Hotel before heading back to the starting point at Scott Farm, but others may not want to have their time dependent on the speed of service at the Doyle.

The Appalachian Trail runs for about 72 miles through New Jersey.

For a shorter run, the section between Delaware Water Gap & Culvers Gap is just over a marathon.

The Appalachian Trail through Connecticut, about 51 miles.