The short answer is that the type of campaign you are hoping to represent, was simply not an option for the period described. Most campaigns lasted for three to four months – between the planting- and the harvesting periods. Soldiers were not ‘professional’ as we know today, but in smaller, distributed communities served multiple roles – e.g. also had to help out with the farming activities.
When the army did campaign, their provisions had to be drawn from the surpluses of the farming yield. However, most farming occurred within a framework of feudalism and was subsistent. Surpluses were rare, and had to be supplemented by trade. Therefore, armies on campaigns often suffered hunger (until as late as the Napoleonic era), and this, (along with the related diseases) was the main cause of casualties.
When on campaign, any army of this period would have three sources for provision: what they could take with them, (see previous point), what they could forage, pillage or barter for, and what they could capture from the enemy. The problem is one of planning – it is generally a good idea to know what you are in for, and not plan on capturing enemy stores, so these were mostly a welcome addition to the current. Logistics for water almost exclusively consisted of taking from the rivers and other sources – especially in Europe, with its abundant sources. Fodder for the animals could be supplemented by grazing – not as effective a food source for the beasts, and it required more time for the animals to feed (and large meadows), but it was possible. 121
Soldiers were often paid a stipend to pay for food, and quartering, and when that was not forthcoming, would simply take what they needed. As such, some parts of Europe which suffered the Thirty-Years War still bear the scars (as do those areas of Italy which witnessed <PERSON>’s invasion – or so I’m told).
That brings us to the primary source – such provisions that the army could take with them. As other responders have accounted, the army needed a baggage train of waggons – these often exceeded the number of the army itself. However, the baggage train was seldom intended to support the army for the duration of the campaign – these merely served as a first echelon. Many of the campaigns for the period followed the flow of rivers, (or hugged the coast), since boats and barges are more effective at the transportation of large volumes than convoys of waggons (and less susceptible to enemy disruption). These typically served as a second echelon from which the first echelon could draw down on. The third echelon potentially offers you the opportunity to make your story realistic – this consisted of a chain of forts, which were also used as depots. The Romans, for instance, would campaign every year for a few months, every time consolidating their gains by building forts at the logical extremes of the campaign’s progress. Any long range venture can therefore be visualised by the string of mutually supporting forts, each containing its own garrison, and of course, stored provisions. For the next year, the Romans were not required to start the campaign from scratch, (or carry everything with them), but they could pick up where they had left off on the previous campaigning season, thereby push in deeper again into enemy territory for the time available. This, along with their engineering efforts at building roads and bridges, ensured better logistics all along the campaign route, except at the ‘tip of the spear’.
How to make this work for your story? You will need a solution to produce a farming surplus, have a professional army (one not involved in the day-to-day survival of the society), have a ‘river-faring’ tradition, perhaps allow some foraging, perhaps even a set of allies on which you can rely, and finally: a multi-year wars, constantly pushing in deeper, and then consolidating gains with engineering works before the onset of winter. 342