It's from wiki of ice and fire. Reference 39 points to Chapter 71 of DWD, Daenerys X.
Nerds are generally pretty good at keeping this kind of stuff accurate, so I assumed this was more or less direct from the book. But yeah if the scale of that map is accurate there's no possible way she could still be on the continent and be thousands of leagues away. Hell, the entire continent of Westeros is probably within 2000 leagues of Meereen, except maybe the furthest northern reaches.
So if she's really only a week or so away from Meereen by cavalry she could definitely kill Jhaqo, take his khalasar, and get back to Meereen in a realistic amount of time.