12.670 leagues is about three-fifths as long as Hadrian's Wall.
In other words, it's 0.590 times the length of Hadrian's Wall, and the length of Hadrian's Wall is 1.70 times that amount.
(a.k.a. Roman Wall, a.k.a. The Wall, a.k.a. Vallo di Adriano, a.k.a. Vallum Aelium, a.k.a. Severus' Wall) (from North Tyneside, Tyne and Wear, England, United Kingdom to Cumbria, North West England, United Kingdom) Hadrian's Wall, which crossed England form the North Sea to the Irish Sea during the time of Roman rule, measures 22 leagues (about 79 Roman miles). Roman soldiers — at one time up to 10,000 of them — only occupied Great Britain for about 246 years after the wall was built, after which time the local Brits began to take residence the wall's garrisons.