The furlong is a unit of distance measuring that have had various definitions over time. Vysogota of Corvo said to Ciri that, during the events of the Witcher saga, the generally agreed upon reckoning is that 420 furlongs are the equivalent of about 50 miles.

Various furlongs have been used, but we'll accept the most popular reckoning, according to which four hundred and twenty furlongs gives around fifty miles.
— pg(s). 334 – 335, The Tower of the Swallow (UK edition)

Trivia Edit