Now that I think about it, did you put enough oil in the tranny? It could be low, or you could be leaking some, which would explain the 20 hours until failure...
Otherwise, sometimes these cars can pop out of gear if the tranny or engine mounts are bad, causing the engine to not sit level...
yes 163k is alot of miles for a stock clutch, but I think my Civic EX has 190k on the original clutch, cause I can tell it's starting to give up...
Here's how to tell your clutch is on it's way out, drive up a steep hill (if you have any) in 3rd or 4th gear, (steep enough to not increase your speed with the gas floored) and watch your rpms, if they start to rise and the car isn't necessarily going faster, the clutch is slipping and needs to be replaced.