If I were to go on a long road trip and had a high mileage car I would say anything over 300,000 miles then I would probably find another car to take on the road trip or rent a car for the road trip.
I've taken a car with 200,000 miles on a 2,000 mile road trip without any problems so I would say that as long as the car has been maintained and looked after and you have things replaced on the car before the road trip then it should be safe to go.
My husband has a truck that has 500,000 miles on it and he has taken it on road trips a lot for his job and he says it doesn't bother him a bit nor does he worry about it.
I myself worry about driving his truck with the 500,000 miles on it just to go 30 miles or so but he's had it maintained and says there's no need to worry.
But for me once the vehicle has over 300,000 miles on it then I would not want to take it on a long road trip because it will likely break down.