It's funny—I had to look up the Catalinas subway station; I wasn't familiar with that one. Apparently, it's relatively new.
Anyway, Calle Florida is pedestrian-only, and for me, it feels like a safe and easy way to walk across town. There are always people around, which helps with safety.
I wouldn't hesitate to walk there until midnight.
Just so you know, you won't reach the Obelisco by walking along Florida. When you get to Corrientes, turn right. The Obelisco is located at the intersection of Corrientes and 9 de Julio.