I think you'll find that high speed internet is readily available all over Japan. They don't have wireless broadband like they do in the US, but most rural areas will have ADSL or DSL.
Here's a link which describes your internet options. It includes a bunch of links to some English language ISPs :
ISP Japan FAQ : ISDN, ADSL, FTTH, CATV Cable, VoIP & Mobile Phones ©2000-8 by Keith Wilkinson