xroadcharlie

I'd at least swap out the tires if they are knobbies for a touring tire, perhaps a narrower one. Google "best bicycle touring tires".

The Schawalbe Marathon Plus HS440 comes highly recommended.

I used an old steel unsuspended mountian bike for road bike duty for a few years. It's stock tires where closer to a hybrid type with a center spine and a small pattern tread so I left them.

I found it too uncomfortable though so I replaced it with a comfort bike. The tall riding position took the pressure off my hands, and despite more weight on the seat, is much more comfortable down there thanks in part to a suspension seat post.

The aerodynamics of a mountain bike aren't that bad, They're probably heavier, But often not by as much as you might think compared some hybrids. For many folks a mountain bike with appropriate tires does a fine job. If you want speed/efficiency, By a drop bar road bike. I had one in my teens, I Have 0 interest in them now.It was unquestionably faster though, especially into a head wind.