How to return from Katahdin is adequately explained in Wingfoot’s Handbook and the ALDHA Companion. So, if you haven’t gotten one of these publications yet, you should have.
In summary what you need to dois:
From the base of Katahdin, usually at Katahdin Stream Campground, ask others (day hikers) there for a ride into Millinocket. It’s not that hard. There is basically only one road into and out of the Park, and it goes into this town.
In Millinocket, there is a hostel where you can shower and spend the night. The next morning, you need to take a taxi to the bus stop in Medway near I-95. The bus is an express bus where you can relax and leave the driving to others. This will bring you to Bangor, where most people change to another express bus for Portland or Boston. In Boston, you can either fly out, or catch a train for points south and west.
If you want to get to Canada, then the shortest route is probably US 201. As you see when you cross it at Carratunk, it’s not that major a road. So, I suspect that if you are flying oversees, then you should plan on departing from either Boston or NYC>
Peaks