@sunflowerinrain this sounds more like an error in how long a horse can gallop than geography. You can easily look up the speed of a gallop and the distance, and conclude that even at a fairly conservative estimate of twenty miles an hour, a two-hundred-mile journey would be a long day of riding, but a doable one.
See also Gandalf leaving Edoras for Minas Tirith (350mi away) at a gallop, and exhorting Shadowfax to "show us the meaning of haste!"