Maybe 'you' can't imagine walking that far, but it's the norm for Infantry, and always has been.
Once they got within their river or depot destination, they walked.
If it rained - they walked.
If it snowed - they walked.
If there were dust storms - they walked.
All of these weather conditions were expected and were a part of their profession.
Cavalry walked a lot, too - especially on long campaigns - saving their mounts for when they were really needed.
Hell, straight-leg Infantry walked over most of Europe during WWII, though trucks were increasingly used, but if the trucks couldn't go further - they got out and walked.
Airborne walked, too - a truism in Airborne is that there are, nor there ever will be trucks on the DZ - after landing, they're foot-mounted and with less personal and organizational equipment than the legs.
There are still Manuals extant from those times (and today - FM-21-18 - Foot Marches - dtd 30 August 2009) that give 'Rates of March' that allow Planners to factor in just how long it might take, and over what terrain they can expect for their Infantry to arrive at the battlefield - and when they do arrive, they're ready to fight.
Scouts Out!