From the time of its founding, the western border of the Shire had been marked by a line of low rolling hills known as the Far Downs. In the east, the border had traditionally been the River Brandywine, though Gorhendad Oldbuck eventually settled a new eastern 'march' (that is, a borderland) beyond the river in Buckland.
In many earlier editions of the Prologue to The Lord of the Rings, Westmarch's dating is given incorrectly as SR1462 (or IV 41). This dating is amended to '1452' in later editions of the book.
Strictly, while it's strongly implied that Fastred was the first of the Wardens, the limited evidence on this point means that some other Hobbit may possibly have held the post beforehand.
In the context of Tolkien's stories, their origins came from the preserved diaries of Bilbo and Frodo Baggins, alongside additional material prepared by Bilbo in Rivendell. Much of this additional material was related to Elvish history and legend, and was presumably the root of The Silmarillion.