I've mainly ridden road bikes, and now have a cross bike for the off-road stuff.
30 miles of dirt concerns me. I've ridden the Katy Trail in Missouri for some distance, well groomed limestone. But, generally not on trails. Especially not for long
Note that I try to get the load behind the seatstays if possible.
That bike is setup with an old Blackburn rack, and the Blackburn Campy rear dropout adapters, and a set of P-Clips (also Blackburn?) at the top. It holds the rack fairly stable, but I do get a little overall flex. Most noticeable when standing and pedaling. One learns to keep the bike in line when standing and pedaling.
I'm slowly working on a new touring bike build, also out of a little more modern road bike.
The Tricross has 35mm tires??? and is a bit more of a rough road
But, even that is falling to a drop bar conversion of an old hybrid (Jamis Coda, double butted 520
Anyway, your old Trek should be able to handle the load (is it a steel frame?). Just get the load mostly behind the rear triangle.
However, are you comfortable riding your Trek on those 30 miles of dirt? Riding it on the dirt with a load?
Some suggestions are to distribute the weight front & rear. My next build.
I'm moving towards lightweight tents and sleeping bags. But the freezing temperatures are a concern for me. 40 to 50 degree sleeping bags?
I could probably tolerate freezing, but wouldn't really like it.