OK
Car is sound
You've been there before (so still need maps etc but some familiarity?)
You are happy on motorways
I assume the PG companion does not require your presence as a potential birth partner or similar (I know, stupid question, but you could be a DH/DP "looking for a pass" in posting).
Check the route beforehand, that you know the main way and a rough idea of the geography near that route in case diversion is required (just in terms of what cities/towns you pass etc, not planning 5 alternate routes in detail).
While doing that planning, DO have a look at where services are, and where you may want to pull off motorway for a break (into a particular town/city that has a nice coffee shop you've heard of, or take a break while wandering a particular attraction perhaps?).
Do have a check of the car the weekend before - tyre pressures, oil, screen wash etc. And make sure you have a full tank setting off.
Pop your rescue organization details, and your insurance details, into your phone.
Make up a bag for the car with snacks (chocolate, crisps, boiled sweets, dried fruit/muesli bars, things easy to eat as you drive), drinks (including some water, and something that has sugar in case you need a boost - i.e. something fizzy, we nearly always have a coke and a 7up/orange in spare), tissues, bin bag(s) etc. Perhaps include either a nice audiobook on cd, or music you love (especially if you can belt out a few tunes to enjoy at top volume!!). If it's soon, also stick in spare socks, gloves, hat, scarf, rug and have a pair of stout boots in luggage or loose - in case of bad weather. An insulated mug, to refill as you leave services stops, can be useful too (so enjoy a coffee when you take your break, but maybe also get one to sip as you drive - or just stop for a "pitstop" (loo break) and grab the coffee to go so you can push on) - it often keeps the drink hotter for longer and is usually less likely to spill etc than the takeaway ones.
And do give yourself plenty of time to get there, and to get home again.
Don't panic, look on it as an adventure to enjoy!!
(A tip an English teacher who was also an aromatherapist gave us years ago was to put 10 drops of Rosemary essential oil on a tissue, put it on the dashboard to slowly diffuse into the car and where you can pick it up to sniff easily - as that is great for concentration! I use it all the time in work (desk based) and for long journeys still).