How did soldiers find their way home after medieval and pre-medieval wars?

Great question! Perhaps the best way to address this is to clarify your underlying assumption.

You mention that medieval (or pre medieval, but that's less my specialty) troops would be drawn from random towns across a decentralised kingdom. The image here is one of lowly peasants levied from their tiny hamlets and thrown together as part of a great army, then essentially disbanded at the end of a campaign.

There are some problems with this assumption, however. First of all, large peasant levies are relatively uncommon in the medieval period, at least in Western Europe. In Britain, Kings more or less stopped utilising a levy-type system within a generation of the Norman Conquest, for instance. The Levy-style Fyrd had been a mainstay of Anglo-Saxon militaries for a long time, but even then the core of any army were professional warriors/nobility who were familiar with campaigning and battle. At Hastings, for instance, it was the Royal Huscarls who made up the core of Harolds army, not hordes of unruly peasants.

With the Norman conquest, armies changed significantly. The mounted warrior became far more vogue, the fyrd system all but vanished. Lords owed the King military service in most instances, and the troops they were obligated to bring was often stipulated in the grants of Land made by the King. The Earl of Gloucester, for instance, owed the service of 400 knights (or mounted soldiers, the definition was not as clear as we'd like). In no way was he required to bring his peasants into service. More often than not the professional forces available to feudal lords were considered equal to whatever military task was before them.

Buuuuuut occasionally non-professional troops were called up, especially for a large campaign. I won't go into the Hundred Years' War, as the contract system used by the English then was quite different.

My go to example is Edward I's campaigns in Wales and Scotland. We have fairly detailed records of what his forces were paid during this time, and they indicate that Edward at various times had thousands of non-mounted troops in pay during his campaigns in North Wales, Scotland etc. We even have information on where they originated. Germane to your question is the fact that many came from quite far away from the campaign Zone.

However it should also be noted that many were drawn from counties and areas quite close to the front lines, so to speak. These men would probably be able to find their way home relatively simply.

Those who did come from quite a distance, for instance a unit of archers drawn from the King's land in Macclesfield, had a distinct advantage when making their way home. they weren't alone. chances were, if you had come from a distance you weren't traveling by yourself. Many men would come with you who either were neighbors or lived in neighbouring villages. Traveling home would be far easier with a group, one of whom assuredly would know the way home given they had found their way to the army muster in the first place

For Nobles, knights or troops of the Royal retinue, your knowledge of the way home would probably be superior just by virtue of having traveled before.

So for the most part, there you have it. As the army disbanded from a campaign, many would make their way back to the army's mustering point. From there they would most likely break up into smaller groups, as troops from different towns, counties, regions etc. Split off to head home. But rarely would a single soldier be required to find his way all the way home by himself, or without knowledge of how to find the way.

Hope this helps! The best source for information on troop numbers and more details for Edward's armies is Edward Morris' the Welsh Wars of Edward I I can provide more sources once I'm off my phone!