How can Offred's story be viewed as a "journey?"

I think that, first and foremost, you need to think about how you would define a "journey."

If a journey is a simple, linear path forward, with no deviations or regressions, then Offred's experience is absolutely not a journey. Before the war, and the rise of Gilead, she was a relatively successful woman with a husband and child; after, she is a slave with no rights, no future, and no clear escape route. She has really lost everything, including her name (she is now "Of Fred"). In the beginning of the novel, she tells us about her age and appearance, as if to try and preserve what little she has left.

However, we also know that the Republic of Gilead does fall, as Offred's story is being recounted to us (somewhat callously) by professors at an academic talk. While we will never know for sure, it seems quite possible that Offred did escape, and that she may have gone on to live a better life. Still, this is not a journey; at best, Offred was able to regain her former standard of living.

If, however, we define a "journey" as personal growth, or as perseverance through adversity, then Offred's experience surely qualifies. Offred lost everything; she was enslaved and abused; she even contemplates suicide at the climax of the novel, when she is taken away by the "Eyes." However, Offred perseveres, and decides to live on in the hope that the "Eyes" are really the resistance. Despite the incredible amount of abuse and indoctrination she has been subjected to, Offred retains her self-worth, and her will to overcome.