In Eltings 'Sword Around the Throne', he mentions supply and logistic officers would often have a member of the Berthier's staff floating depots around organising drifters from various units that had fallen out to catch up with their regiment.

It is never explicit but many accounts of the soldiers I have read seem to indicate they had a fair idea of where they were headed so even if they had to fall out enroute due to illness/exhaustion they would at least keep heading towards the next stop. In Marshal Jean Lannes biography, the author paraphrases Berthier's orders as "you need to be at town X on the night of Y, no later than Y+n days). How this information was disseminated to the individual isn't clear but military custom would suggest that the word to stike camp/bivouac and head to X would be the first thing passed down.

In Jean-Roch Coignet's diaries, he writes that maybe Napoloen could form his regiments based not on height or function/armaments but on their ability to tirelessly march to keep up with the advance guard and cavalry.