I'd borrow heavily from Night's Black Agents here. What you're running is a thriller, where the PCs are, in a way, in the position of Jason Bourne. Very capable on their own, but outclassed by an enemy that keeps coming out of nowhere, and if they show up with great numbers, it's all over.
First piece of advice: have a scene where some capable bystanders are utterly and thoroughly destroyed by the hunters. Or even better, have that happen off screen, to prevent the PCs from wading in to a TPK.
Night's Black Agents suggests that there are only two types of scenes - information gathering and action.
So the first thing for this is "gather information." In this case, if they come across a dismembered, disemboweled, folded, spindled, and mutilated battleground, where the losers just happen to resemble the PCs to some extent. This one was brown-haired and wearing mail...just like Bog. That one was fair haired with a bow and leather scale. That's not quite Betterthanyouiel, but it's close enough. Geez, fatal case of mistaken identity!
The tracker could say they were swarmed over and overrun. The point guy of the dead group is in two pieces - but only evidence of one blow (gulp - if they hit us, we're dead!).
So there should be some fear there of individual beasts, as well as a pack.
Then you can stage minor skirmishes (action scenes) where if things go well they escape or can deal with a minor scout threat (a lesser beast?). That's the action bit.
The investigation is (a) why are we being hunted? (b) What's hunting us? (c) Do we fight, bargain, or run? (d) Do any weaknesses exist? (e) Do we need to go on adventures in order to obtain what we need to take advantage of those weaknesses? and finally (f) how do we set it up so we can kick their butts by using clever tactics and leveraging their weaknesses?
If there's an action scene of some sort in between each question, that's at least 12 sessions right there!