You could always run into conveniently un-travel-in-able weather, or trip over a kid in the road...
Or you could have one of the horses in your group trip over a woodchuck hole, lame himself, and kill his rider and then have to spend a bit of time... I dunno... bringing him back to Edoras rather than lugging around a dead body?
Then again, something like that would probably mean that only one or two of you would be sent back, not particularly slowing the others down.
*ponders*
What if Sorn were to hear that the Queen had sent a group of rescuers out rather than somebody with a nice fat ransom and feigns a letter from her calling them back due to something important ("The package that you were sent to pick up has been delivered safely." or some such) and sends a message-rider out for them? The rider could disappear once they reached Edoras and the mistake could take a lovely amount of time to sort out...
Say they're riding for a day when they get the message calling them back; that's a day's ride back. Then sorting out the confusion. Then the ride back to Sorn's during which he will have moved Lin?
It's a rough sketch, but it could give you time while adding a nice bit of conflict... Everybody likes a nice bit of conflict...