Google Maps thinks it's 23 miles from Newburyport to Kittery via I-95. My proposed route is to just go right up the coast, with one very scenic detour ("follow MA-1A to NH-1A to NH-1B to US-1 to Maine"). I suspect the total will come out to around 50 miles but don't know for sure. I've driven the route recently and don't remember it being doomfully hilly at all.