While not strictly linear, it felt (at least on the harder difficulty) like it was an intended path through the game, as most areas would be blocked by enemies that were too powerful to fight.
I'm playing on hard also. Yes, there are a lot of areas that feel blocked off in the first half of the game due to enemies, but I still felt like it really opened up in the second half.
It's not perfect, but it's better than if the enemies scaled to your level.