Horses run on the roads/sidewalks so I don't see a problem. It's getting around/between lots and objects.
Yes, they won't be able to go into that red area. But make sure there aren't any holes in the object area. Some objects, I can't think of any right now- don't restrict access all the way through (like a hole in the middle. Which makes a place Sims can generate and get stuck.
Turn off Objects in Toolbar-View-Layers and untick Objects. That will hide all the objects so you can see under and through them. :)
https://i.imgur.com/XkNJvy0.jpg