I did manage to at least start out with some description (though I lost it quickly as we went into combat). This helped set the mood a bit, and I noticed that people were speaking in character much more than usual.
The combat itself was pretty fun. The barbarian bugbear managed to impress them with how much damage it was able to soak up. They still ended up taking almost no damage that they couldn't heal the next day, but at least they got scratched. (Note to self: if having only one encounter per game day, the EL needs to be a couple of levels above the party to be interesting.)
Unfortunately, I don't seem yet to have created an atmosphere of urgency. In particular, the party is currently going through a prolonged and tedious process of figuring out how to transport their mounts across the shattered bridge, despite the fact that they've concluded that the Duke they're trying to rescue is in trouble.
I guess I'll try to remind them of the urgency when they recap next, and warn them that this may not be a prudent use of their time. And if they don't make it in time... well, it seems like a waste of a good duke, but I'd do it to make a point.