The correct answer is “only in summer is sea fish carried to markets that far inland”.
Indeed, during that era the road infrastructure was very rudimentary and underdeveloped. There were not as many roads as there are nowadays and such roads were not paved or asphalted. Transportation was also rudimentary, simple cart wagons hauled by horses and considering how cold it can get during winter time and how much it can snow it was quite dangerous to venture on a delivery run with such primitive means.
