I feel like not going on a trip.
Is there a reason for that?
I just don't want to be in a car for long distances
You don't have to go on a trip you don't want to. Maybe travel via a different way.
I plan on letting him know I don't want to go.
There's no reason to feel bad if you are not feeling up to a trip. But it would definitely be good to let them know beforehand that you feel like not going.