I think that the victory points for the encounter correspond with the distance. Each encounter in the first ring of provinces around your city gives 1 victory point. In the second ring it's 2 victory points. And so on. Therefore the fifth ring of provinces around your city has encounters that give 5 victory points. That should help you to identify the distance.
PS: If I remember correctly, this is very hard to do when that quest shows up. If you can't do it, just leave it for later (or just decline that quest, if it's "declinable" and if the rewards are just coins+supplies).