You want a bike that

1. Fits you
2. Is sturdy enough to endure many kilometers of loaded riding
3. Is in good enough condition for same

Many older mountain bikes work just fine for that. Others don't. Depends on what you've got. Give some specifics about your bike to get some opinions on that (brand of bike, frame material, size of wheels, number of spokes per wheel, type of gearing, brakes, etc.). I and many others tour on older mountain bikes that work just fine.