Since I want a relatively long overland adventure I needed a reason for it . . . so I came up with returning a lost young centaur to his family.
As for the "escort" I decided that they would be young adventurers who had been shipwrecked and who were saved by Abelard de Amber, the ruler of a tower on the barren coast. He will charge them with safely delivering young Zilwen to his family . . . who are somewhere around the Lake of the Crown Beast, which is deep inside the Glow-Worm Steppes.
Finally there is the problem of how Zilwen ended up several hundred miles from his family.
Aha! Here's where we get to introduce an as yet unknown enemy. Zilwen was in the process of being kidnapped when he triggered a magical device that teleported him to this remote coast. At some point (much later in the adventure), this enemy will become aware of their quest . . . heh, heh, heh.
So this is the setup prior to the start of the adventure.