I don't do any technical mtb riding, but I do ride off-road on mild singletrack and towpaths. The biggest differences I notice when switching from pavement to off-road are in the vibrations and rolling resistance. Even a relatively smooth trail is going to have more resistance than a paved road, and probably enough unevenness to cause more vibration through your hands and seat. Also, I'd guess that the hard-tail you'll be using might be heavier than your road bike. All of that could add up to more fatigue than you're used to with road riding.

Is this a supported trip? Will someone else be doing all the work to set up camp and cook so that you can relax and rest at the end of each day? If so, you may be more tired than you'd be from an equivalent distance road ride, but you could probably handle it if they feed you well