OK, I'm going to go out on a serious limb here--I'm going to postulate that Lucas likely read William Morris's 1896 fantasy novel The Well at the World's End. It would have been easy for him to get ahold of, as there was an Ace paperback edition released in 1970.
Morris's story opens with King Peter of Upmeads, who asks three of his four sons to go out into the wider world, so that they might learn skills and prove their valor, and so he might see which one is worthiest to succeed him.
The fourth and youngest son, Ralph of Upmeads, is the protagonist. Peter tries to keep Ralph at home, not wanting all his sons to leave him, but Ralph runs away to have an adventure of his own. It's no surprise that at the end of the story Ralph is the one crowned King.
As he travels Ralph learns of the Well at the World's End, a magical fountain in a far-off land that grants long life and strength of will to whoever drinks from it. He decides to seek out this Well.
Morris, like Fritz Lang, uses the Madonna/whore dichotomy in symbolizing his two principal female characters. The Madonna is the Lady of Abundance, a sorceress who is presented in terms of a Marian goddess figure. Ralph falls in love with her, but she is killed by a jealous ex-lover before they can consummate their relationship.
During his travels Ralph also meets a peasant girl named Ursula, who becomes infatuated with him. Initially he spurns her, still in love as he is with the Lady of Abundance. However, after the sorceress dies, Ralph dreams of her, and she tells him to seek out Ursula.
Ralph sets off in pursuit of Ursula, but soon learns she was captured and sold as a slave to the Lord of Utterbol. Ralph himself is enslaved by this evil lord, but manages to escape before long. Ursula also escapes her captors, having stolen a suit of armor. However, Morris later implies that she was raped by both the lord and his nephew during her captivity.
The two meet up and set out together for the Well at the World's End. Pursued by the troops of the Lord of Utterbol, they manage to traverse a barren maze of rocky paths that bars the passage over the mountains, guided by a wise man known as the Sage of Swevenham. After that they come to a pleasant valley. (Ralph and Ursula do not get to take revenge on the Lord of Utterbol; that honor goes to one of Ralph's friends whom he meets on the way.)
One day, while bathing in a lake, Ursula is chased by a hungry bear. Ralph sees her fleeing, naked, for her life and kills the bear. Afterward, Ralph admits that the sight of Ursula naked made him realize that he loves her. Soon thereafter they find a village of "innocent folk" who marry them in a pagan rite.
Before they reach the Well at the World's End, they must cross a vast desert. In the middle of this desert is the Dry Tree, an ominous withered tree with poisoned water about its roots. The tree's evil magic bewitches men into drinking its water; Ralph is only saved by Ursula's warning.
The duo finally find the Well at the World's End (sunken into a coastal cliff, it is only revealed at low tide). They drink and feel themselves imbued with new vitality.
The remainder of the book concerns Ralph's journey home to Upmeads, where he routs a gang of bandits that have taken over the kingdom in his absence. His father Peter, recognizing Ralph as the best suited of his sons to rule, abdicates, and Ralph and Ursula become the new king and queen, living for an extraordinarily long time.
This story was obviously a major inspiration for J.R.R. Tolkien--and not just in The Lord of the Rings; the Silmarillion narrative of Beren and Luthien also owes Morris a hefty debt. Things like Frodo's captivity in Cirith Ungol, and the Scouring of the Shire, have their origins here.
Lucas likely also read Morris's novel; its influence shows in several of his works.
The gang-rape Leia Aquilae goes through offscreen in the 1974 rough draft is strikingly similar to what happens to Ursula--mediated, of course, by the prism of Tolkien, who also borrowed from Morris in this regard. Lucas follows Morris in applying his rape narrative to a woman, as opposed to a male hero like Tolkien did. But Lucas borrowed from Morris in other ways as well.
Consider the Dry Tree: a poisonous tree in the midst of a vast desert wasteland. This image appears to have recurred in early story concepts for Willow, where a sinister tree apparently sat in the middle of a vast desert. In Lucas's version, the tree was sinister because it marked the entrance to the cave of a hungry dragon.
The idea of a barren rocky maze--in Willow surrounding the lost castle of Tir Asleen--likewise appears to owe something to Morris. Fin Raziel, the sorceress, assumes the function of the Sage of Swevenham in leading the main characters safely through the stony labyrinth.
As well, Willow's Madmartigan first falls in love with Sorsha when he sees her sleeping in her tent--and originally she was meant to be sleeping naked. (The final film put her in a nightgown.) This is reminiscent of Ralph first realizing that he loves Ursula when he sees her running naked from a bear.
Lastly, there's the fairy-tale motif of the youngest son having to prove himself the equal (or better) of his older brothers. This shows up markedly in the 1975 second draft of SW, where Luke is one of the youngest sons of the Starkiller family of Jedi, and must prove to his father and brothers that he is a worthy son--in fact, fated to be the worthiest of all.
And in the third draft, although Luke's large family is gone, Luke runs away from his home and his foster parents to seek adventure in the wider world, much like Ralph does.
I have to say, I have a newfound respect for Lucas, sheerly in terms of literary scholarship, seeing that he pursued Tolkien to his sources like this. Not many people would have made the Morris connection in the 1970s; few more do now.