Get some fenders and take care of the drive train once you're done, all will be well. I recommend fenders because they will greatly reduce the amount of grime and grit that ends up on you and the bike. I have Crud MK2 fenders and love them because the rear fender is designed to protect the drive train, after having ridden in the rain without fenders I can tell you it is much better having them than not.

Cut a piece of old tube and put it over the seatpost clamp (you have to remove the seatpost). Secure both ends with plastic cables ties nice and tight; that'll keep water out of the seat tube.

I use a rear Race Blade... I'd use the front one but it doesn't fit my bike. The rear is more crucial, esp for keeping your rear end dry. The front one I don't think is as crucial since the down tube ends up deflecting a lot of it anyway. But if you have someone pacing behind you, they will be supremely glad if you have a rear fender on, I can say from experience.

My wife is considering taking our hybrids instead, those have fenders etc. Personally I think she just wants to ride her newest acquisition. :)

We're mostly going to be on the trail, the Little Miami, which in total is a 70 mile paved trail, though we're not doing the whole thing. We were going to take the road bikes to get as much advantage as we can because we've ridden with one of the people that will be with us before, and she dropped us.

Chuckles, that's what I've been told on several occasions. Sometimes because it is a Trek at all, or because it isn't a Madone, though the goofiest one was because it's only a 54. I've come across some odd types. The guys on their aero bars can pass me all they want on the trail, I'm just in it for the fun not training to become the next Merckx.

Watch the painted white or yellow lines on roads and streets. They can catch you
off guard. From experience. ouch!

I'm trying to imagine living in a place where rain is such a rare occurrence that it requires altering one's plans.

It only rains a couple handfuls of days per year in Tucson. But I don't alter my plans because of it.