diuturnal (dee-oo-TUR-nuhl) - adj., lasting a very long time.
olamic (oh-LAHM-ik) - adj., everlasting, eternal.
Both of these, as you might expect, are marked "now rare" in dictionaries. The former seems to have been borrowed by Milton from Latin diuturnus, from diu, which meant both a long time and a day, from dies, day. Since as we all know, a day can stretch out forever. The latter, on the other hand, is from Hebrew olam, an age, eternity -- I'm not sure when it was coined, though. I'm also not sure whether that's the same olam as in tikkun olam, "repair the world."
The old rabbi's homily on the importance of tikkun olam was diuturnal -- nay, olamic. I thought services would never end.