I think there's more than meets the eye that will later pop up. I can see both sides as to or not to leave during the middle of the season. Either way, he has left a legacy.
He likes to drink (has been drunk during a post-game show at least once that I know of). That's about it. But I've seen pictures of him this year exercising with players and all of that, so unless something has just come up recently, I doubt he's suffering from any health effects other than heartache / butthurt at how this season is going.
From what I read, he's going to be on Gameday tomorrow, and I couldn't see him doing that if he had any serious health issues. And I bet he's coaching next year or the year after, regardless of what he said in the press conference. Hell, he could take that North Texas job and win a Sunbelt Conference title.