Well, I don't know what they were up to, but it seems unlikely to me that they would fly that combination of aircraft in formation, or for so long, as part of a tactical exercise. If they had an exercise of some kind, they would normally just fly over on the way to a block of airspace off shore which is there for that purpose. I think perhaps they were rehearsing for something else. Air Festival?
We enjoyed two air festivals in 2007. Last year, they didn't hold one at nearby Hyakuri Airbase due to construction on the field. So I searched the "internets tubes" for the 2009 calendar of the JASDF aerobatic team "Blue Impulse" and discovered that there is indeed an air festival in the works for Hyakuri this year. It will be held on Sunday, September 13th. As that is close to my birthday, I'll make it a present to myself.