If you read any of the books written to describe the experiences of any of the German contingents fighting alongside the French in Spain, you will see that daily life for the soldiers consisted of taking post in a garrison (large or small), fortifying it (the inhabitants inevitably fled at their approach) and searching madly for something to eat and drink.
From then on, time was taken up with being sent out on foraging expeditions to collect food, in which every peasant seen was the enemy, armed and murderous.
Large expeditions found no-one, small expeditions vanished.
As each surrounding village was looted and burned, so the peripheral environment of every garrison became a barren, hostile desert.
There was no re-supply of rations or forage from French sources, small garrisons dwindled away from disease or starvation, or were picked off by the local guerrillas, who often attacked the fortified garrisons in force.
The ignorance about life for the French and their allies the peninsula stems from the fact, that many admirers of N won`t bother to read about anything which existed out of HIS range of vision or earshot. Thus, most know Esposito and Elting`s excellent Atlas, but as N was only briefly there once, and the Atlas coverage of that campaign is very broad-brush and relatively very brief (only 8 maps out of 156 and Marengo got 9), their knowledge of the war there is very superficial and their interest scarcely encouraged.
The scarcity of food and forage in Spain and Portugal and the lack of any central re-supply, meant that large proportions of the available military force had to be dedicated to daily survival duties. It also meant that orders and reports often got snatched by the guerrillas and that any large force, brought together to combat Wellington, could only remain concentrated for a very short time, as their food ran out and the guerrillas quickly entered the rear areas that they had evacuated, attacking their garrisons.