Origins
The word 'journey' entered Middle English in the thirteenth century from Old French 'jornée' (a day, a day's travel, a day's work, a battle — since a battle was a day's work for a soldier). The Old French form derives from Vulgar Latin *diurnāta (a day's portion, a day's length), built on Latin 'diurnum' (daily), from 'diēs' (day), ultimately from PIE *dyew- (sky, heaven, day, god — the same root that produced 'deity,' 'divine,' 'Zeus,' and 'Tuesday').
The original meaning in English was precisely 'the distance one could travel in a single day,' typically about twenty to thirty miles on foot or horseback. This was not merely a metaphorical convenience but a practical unit of medieval measurement. Land grants, legal boundaries, and pilgrimage guides routinely specified distances in days of travel. The shift from 'a day's travel' to 'any act of traveling, regardless of duration' was well underway by the fourteenth century and was essentially complete by the sixteenth.
The family of words spawned by Latin 'diēs' through the French intermediary is remarkably large in English. 'Journal' (a daily record, from Old French 'jornal,' daily) entered English in the fourteenth century. 'Journalism' followed in the early nineteenth century. 'Adjourn' (from Old French 'ajorner,' to set a day for, hence to postpone to another day) appeared in the fourteenth century. 'Sojourn' (from Old French 'sojorner,' to stay temporarily — literally 'to spend a day under,' from Vulgar Latin *subdiurnāre) came in the thirteenth century. 'Diurnal' (daily) was borrowed directly from Latin in the fifteenth century, bypassing French entirely.
French Influence
The word 'journeyman' preserves the original 'day' sense most faithfully. A journeyman is a craftsman who has completed his apprenticeship and is qualified to work for daily wages — from Old French 'jornée' (a day, hence a day's wages). The word has nothing to do with traveling, though folk etymology has long encouraged that association. The three grades of the medieval guild system — apprentice, journeyman, master — encoded a progression from unpaid learner to paid daily worker to independent practitioner.
The Romance cognates of 'journey' have diverged interestingly in meaning. French 'journée' still means 'a day, a daytime period' as its primary sense. Spanish 'jornada' means 'a working day, a day's march, a stage of a journey.' Italian 'giornata' means 'a day, a day's experience.' Only English has fully detached the word from its diurnal origin, allowing a journey to last weeks, months, or a lifetime.
In modern figurative usage, 'journey' has become one of the most productive metaphors in English. A person's life is a journey, a career is a journey, healing is a journey, learning is a journey. This metaphorical extension — life as travel — is not unique to English; it is one of the foundational conceptual metaphors identified by cognitive linguists, present in virtually every human language. But the English word 'journey' carries it with particular force because the word itself has already traveled so far from its origin.