The good news is that they're going very fast, and if the sun is following its usual course, they're even headed in more or less the right direction. Well, assuming that Dean's correctly interpreting the rumors he'd overheard, but a giant with long hair who's locked in a tower does kind of sound like Sam, and it's not like he has anything else to go on.
He's gotten almost used to the whistle of passing branches almost taking his head off, so he doesn't realize something's going on until the second arrow flies right past his nose--and then there's a rope coming for him right at face-level, and all he can do is cling to the horse's neck and hope he doesn't get his own broken.
"Stand and deliver!" Someone shouts over on his right, and even as he turns to look he can see something coming at him from his left, but it's too late.
Getting hit feels like falling sideways, and then he actually is falling, for a breath-stopping moment, and then the earth hits him going in the opposite direction. He can hear his head bounce, which seems like a bad sign, and then everything goes very dim and far away for a little while.