And the Big Little Lies star admitted making the film changed her forever after pushing her to her limits.
She said: "I don't know if I'll ever work that hard again, but it changed me on a cellular level."
But Witherspoon - who has Ava, 21, and Deacon, 17, with ex-husband Ryan Phillippe, as well as Tennessee, 8, with spouse Jim Toth - thought Strayed's book was "beautiful and sacred" because it had taught her a lot.
She said: "[It] was so beautiful and sacred to me because it spoke to me so deeply about how we as women have to save ourselves. There's no mother or father coming to save us. There's no spouse.
"I thought it was radical that at the end of the film, she ends up with no family, no money, no job, no partner, and she's happy."