פּירוש אויף יונה 3:4

overturned Destroyed. [He did not say, “Destroyed,” because “overturned” has two meanings, bad and good. If they do not repent, it will be destroyed; if they repent, then “overturned” refers to the people of Nineveh, that they will be changed from bad to good, and they will repent. Gloss of Rabbi Akiva(?)]

And Yonah started to walk in it [the city] for one day's walk and immediately its people were roused to do repentance, even though in Bereishit Rabah they said that Nineveh was a forty-day walk, maybe that was [describing the distance] in the outskirts of the city, and therefore the decree was: "Forty more days, and Nineveh is overturned" because there were forty days [required] until the announcement was executed in the entire land- the city and its outskirts, and [only] then Nineveh could be destroyed. This decree was certain, without any conditions at all.

"And he started"- Some say: