3:3 Ninevah, a 3-day city

What does it mean in 3:3 when it says Ninevah took 3 days to visit?
No time to mention this tomorrow, so I'll mention here:

three days’ journey in breadth (cf. esv footnote, “a visit was a three days’ journey”). In Jonah’s day neither the circumference nor the diameter of the walled city of Nineveh (see plan, p. 1691) was a three-day walk. The phrase may refer to the time it would take Jonah to walk throughout the city, preaching his message. (Nineveh could also refer to the much larger administrative area including the city and the outlying villages, which was 30–56 miles/48–90 km across.) --from ESV study bible