day's journey; one day's travel
Refers to the distance one can travel in a single day. Somewhat literary or old-fashioned; in modern conversation, 一日の旅 or 一日の道のり is more common.
From here to the next post town is a day's journey.
Compound of 一日 (いちにち, 'one day') and 路 (じ, 'road, journey'). The reading じ for 路 is a common suffix in distance-related words.