114.86 km (71.36 miles) - 'in the city of Saku':
Nagano is an inland prefecture and it borders more prefectures than any other in Japan. Nagano contains the point furthest from the sea in the whole of Japan - this point lies within the city of Saku.
https://en.wikipedia.org/wiki/Nagano_Prefecture
Saku holds the distinction of containing the point furthest from the sea (actually this point lies within the former town of Usuda).
https://en.wikipedia.org/wiki/Saku,_Nagano
Usuda was a town located in Minamisaku District, Nagano Prefecture, Japan.
As of 2003, the town has an estimated population of 15,731 and a density of 189.05 persons per km². The total area is 83.21 km².
On April 1, 2005, Usuda, along with the town of Mochizuki, and the village of Asashina (both from Kitasaku District), was merged into the expanded city of Saku.
https://en.wikipedia.org/wiki/Usuda,_Nagano
The most distant point is located in former Usuda Town, Nagano Prefecture, whose distance from the sea is 114.86 km
http://poj.org/problem?id=3525
