Wow, where to start. Journey is a good theme to discuss with this novel, but journey can be expressed and explored in many different ways where McCarthy's story is concerned, so let me give you a few quotes and ideas to explore, along with some key passages that reflect this theme.
Can I ask you something? he said.
Yes. Of course.
Are we going to die?
Sometime. Not now.
Life is a journey too, even in the best of circumstances. Life in The Road is not the best of circumstances, by any means, but the boy's father is quite certain that he's not done living yet, that some of his life journey is yet to be played out.
It took two days to cross that ashen scabland. The road beyond fell away on every side. It's snowing, the boy said. He looked at the sky. A single gray flake sifting down. He caught it in his hand and watched it expire there like the last host of christendom.
There is literal, physical journey: the long march through the days of attempted survival, trying to reach the ocean. I like this passage because everything, the landscape and the seasons themselves seem to be just barely hanging on to the edge of existence. It is one of the strongest themes of the story, in my opinion.
He walked out into the gray light and stood and he saw for a brief moment the absolute truth of the world. The cold relentless circling of an intestate earth. Darkness implacable. The blind dogs of the sun in their running. The crushing black vacuum of the universe. And somewhere two hunted animals trembling like ground-foxes in their cover. Borrowed time and borrowed world and borrowed eyes with which to sorrow it.
This is a mental journey. The slow, relentless realization that all is for naught. It is the journey his wife completed before him, and that led to her death by suicide.
There are certainly more to choose from, but that will get you started.