。
The post office is half a mile farther on.
Literal
Post-office [topic-は] half-mile beyond [at-に] exist.
マイル ('mile') is a katakana loanword; Japan uses the metric system in daily life, so miles are a foreign unit mentioned mostly in translations from English. 向こう ('beyond, over there, the other side') is a versatile spatial noun for 'past that point.' Note that 半マイル uses 半 as a prefix meaning 'half' — parallel to 半年 (half a year).