You have 6 hours of driving ahead and a 1 year old dog whose longest trip so far is the 10 minutes to the park.
This one is largely workable in advance. The dog who travels well on the day is usually the dog who practised beforehand.
Practise before the journey, not on it
Working below the threshold at which your dog reacts is the condition for learning. If they pant, whine or scrabble, the last step was too big.
Sit in the parked car with your dog for 3 minutes a day this week, feeding them in the spot they will travel in.
Add 5 minute drives that end somewhere ordinary, like your own street, so the car stops predicting only the vet or the park.
Build up to one 45 minute drive before the long one, so the day itself is not the first long trip.
Shorten the next session rather than pushing on, any time your dog cannot settle.
Set the car up
Secure your dog properly, in a crate that is strapped down or a harness with a seat belt attachment.
Put a familiar bed or blanket in, so the surface smells like home rather than like a car.
Cover the window side of the crate, so traffic is not moving past their face for 6 hours.
Ask your veterinary team about the meal before you leave if your dog has ever been sick in the car.
On the day
Walk your dog for 20 minutes before you load them, so the first hour is easier.
Stop every 2 to 3 hours. Leash on before the door opens, 10 minutes of sniffing, water, then back in.
Keep the car cool, and never leave your dog alone in a parked car.
Hand out a chew at each restart, so settling back down has something to do.
If your dog cannot settle in the car
Relaxation is trained as a behaviour in its own right, in short repeatable sets below your dog's threshold. The parked car is one more place to run those sets.
A dog who has been carsick or frightened in the car needs a plan before a 6 hour journey. That is a conversation with your veterinarian, not something to drive through.
Safety note
Speak to your veterinarian before a long journey if your dog drools heavily, vomits, or panics in the car. Sudden behaviour change warrants a veterinary examination. Never leave your dog alone in a parked car, in any weather.
Today's version, in one line: 3 minutes in the parked car with food tonight, and a 5 minute drive tomorrow.