For me It’s been quite the opposite. Leonbergers love to be out in the woods, lay in a creek, and run up or down a hill.
I’ve only taken my Leo on one major mountain before and that was Hanging Rock in North Carolina when I was down there for a few months. The weather was hot and it is a bit of a tourist attraction; so a major tip would to bring an abundant amount of water, and I mean probably more than you think is necessary. And though my Leo wasn’t scared, when we did take a break to sit or her to drink she didn’t seem so comfortable with people coming up and down the mountain seemingly out of nowhere. But that is just my dog, another might love seeing all the people.
I had her leashed the whole time we were walking but at two small waterfalls we came across she was off leash and able to play in the water, it all depends on how comfortable you feel about having them off leash, if you think the dog is ready for it and you’re confident they’ll come when called. I know my Leo is most distracted when on the beach, It’s probably her favorite place and I take her at night so there’s almost no people and she can be off leash.
Now on back woods trails or through a field my Leo can’t get enough, especially if said trail is covered in snow in New England.
I’d say to definitely take your dog on that hike, just always be mindful of how the dog is doing. If they’re panting and the tongue is a little too white or they’re holding head pretty low, take a break. But a Leonberger loves to run around in the woods and be outside, and will definitely be passed out on the car ride home.