I recently saw him at the Emily K Mother's Day Ball (a fundraiser for the Emily K Center), and he looked about as spry as ever. I'm not discounting what you wrote in saying the above nor implying that walking around a ballroom and up some stairs to speak from a podium is exactly the same as the activity involved with coaching. Just that, based on what I observed, his gait and overall "health" appeared just fine that evening.
I remember seeing that Coach K had ankle replacement surgery (or some kind of reconstructive surgery) during last season. So maybe he was having issues with it during his final season and finally had something done about it and that's why he looked so spry at the Emily K event.