An eternity that is far away is not really an eternity, it just feels like it. It's forever, but not the real kind of forever that's actually forever. That's 永久, which is a real forever, this is more like "whoa, I haven't eaten sushi in forever" (actually, you have).
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.