This is an ally quest, where you take the quest mount up behind the guy you free´d and then tosses fire and other stuff on the worgen that will try to kill you.
This used to be a fun and easy quest before cata, not sure what they did, but i have never failed this one before the 4.?.? patch, but after they made it harder.
What to do then? well you do as told, mount up as normal, but then you quickly mount off again, then takes your own mount and then just follow him, the guy will go on just like you where on his mount and no worgen will ever spawn untill you mount up again, and that you have to do right after he crosses the second river, but then the horse will have full health and you cant lose.
Note: you will fail the quest if you dont mount up before he gets to Amberpine Lodge. Just ride up next to him, you will get a green arrow to jump to his horse, you dont have to demount.