Riding packed limestone trails on 700x23 ties I recently bought a Trek 2.1 and it came with 700x23 100psi tires. I do a lot of road riding, but every once in a while the packed limestone (rails-to-trails) trails are the better option. I've ridden a few times already with no incident, but I would like some input as to whether this is a safe idea?

Side note, I am planning a self-paced century that will utilize almost 50 miles of packed limestone. If 700x23 is not a great idea, what width would you recommend for a limestone/road century?
You may want to go up to 700x25s or 700x28s. While you should be okay with 23s, you'll definitely have greater peace of mind with a thicker tire. You may also want to drop the pressure slightly to 80–90psi.