I'd say you can ride it pretty much anywhere ... speed dependent on terrain, of course.

The original mountain bikes were old cruisers with fat tires ... just cheap steel-framed trash bikes that a few enterprising Marin County residents decided would work just as well and be more fun out in the woods.

For that matter, how many of the older posters didn't venture into the woods at least a few times on their old steel tanks back in the day?

Gearing and traction (tire width and design) will limit your climbing ability; bouncing around will limit your descending speed. Roots and rocks---the shock of hitting them---will slow you down on the flats.

Otherwise, you could ride full-on mountain bike trails ...

I sometimes photograph local MTB events, and there are guys riding rigid-framed single-speeds for 8- and 12-hour races. I am sure they use stronger and heavier wheels, but otherwise they are just riding bicycles. If you plan to ride serious trails, I'd recommend saving your road wheels and buying a wider set to swap in.

Only way to tell what you can ride is to go out and try it. You won't break the bike unless you crash.