I'm really in awe of the people here claiming to have toured on 25mm tires, and I'm even more surprised that nobody here is raising a red flag when someone is talking about TOURING for A WEEK, on UNPAVED ROADS with 25C tires. I see I've arrived too late, and that the original poster has already purchased tires (good choice on the Schwalbe Marathons). I did just check the Specialized website, and the Sequoia Elite seems to have been designed to not accept wide tires, so maybe that is the limiting factor for the original poster...but I'll rant anyway...

I guess if you're a small guy touring on very smooth roads without a load and a bicycle incapable of fitting anything wider than 25c, then 25c tires would be okay. On the other hand, it sounds like he's going to have a very harsh ride. Depending on the conditions of the "unpaved" road, the weight of the touring load he's carrying, and the speed that this rider will be going, this could be anywhere from terrifically uncomfortable to frighteningly dangerous.

Am I wrong? Lets argue about this.