According to a semi-canon source, Hayford is located half a day's ride north of King's Landing along the kingsroad. The hidden village of the Lady of the Leaves is located near the Hayford road.
Levies from Hayford joined the host of Lord Borros Baratheon during the Dance of the Dragons, but they abandoned him during the Battle of the Kingsroad. Hayford sent levies for the host of Ser Robert Rowan which marched to aid Ser Joffrey Arryn in the Vale of Arryn.
A Feast for Crows
During his journey to the siege of Riverrun, Ser Jaime Lannister camps his forces by the stream beneath Hayford's hill. When he is asked to supper by the castellan of the castle, Jaime brings several lords and knights with him to the dinner. Young Lady Ermesande Hayford is briefly presented to the guests, but the toddler is taken away when she begins to cry. The castle's castellan asks about Ermesande's spouse, Lord Tyrek Lannister, but the youth remains missing after the riots in King's Landing.
Unable to fall sleep in his tower room, Jaime goes in search of Ser Ilyn Payne. Under the moonlight, the one-handed Jaime spars with Ilyn using blunted tourney swords in a small courtyard next to Hayford's armory. When Lewys Piper asks about Jaime's bruises the next day, Jaime tells his square they are love bites from Hayford wenches.