I can't believe no-one's mentioned this, but make sure you have basic tools in the boot (or trunk as you call it
)
It's no good having a spare on you if you remember at the crucial moment that you left your jack in the garage!
Also get some radiator sealant just in case, the most likely thing to happen to an older car on a long journey is your stressed radiator or hoses will spring a sudden leak. Remember, if you overheat, let the engine cool down before removing the radiator cap.
Spare headlight bulb may be handy (unless your headlights are sealed units which can be a pain in the arse).
Now is the time to sort out any odd rattles or knocking noises you've been wondering about for a while.
Check oil, water, tyre pressure, fan belt, that sort of thing.
800 miles is it? Did road-trip (as a passenger I hasten to add) of 4000 miles each way once - London, England to Alba Iulia, Romania - and back again a month later. Was great, I love travelling!
Above all, have fun.