To earn this achievement you cannot gallop, that is sprint with your horse, until the ninth day has started. That means you cannot use
,
, or quickly double-tap
or
. I recommend doing this on the first island as you won't need to venture far and most resources should be nearby.
Once the ninth day has started, that is, you read IX on the screen, feel free to gallop to earn the achievement. It will only unlock once you galloped.