If you're efficient going through the story and arcade, you will actually have to leave your xbox on to get the 20-hour achievement. I just finished my last achievement other than the 20-hour and the 2nd playthrough and I'm only at 14 hours total playing time. I wasn't even as efficient as I could have been in story mode either. Since it doesn't carry across save files, I guess I'm leaving my xbox on today while I sleep.
400,000gs reached 1-13-15. Used D4 to get it (get it, it has 4 in the name) due to my disgust with how Halo: MCC has been handled. Achievement used for 400k milestone: Business as Usual.