I'm at 40 hours and just entering act 4. So yeah it is a decent length RPG.
For Karthal, you must enter for the first time through the main story quest.
To enter the lost city entrance in seahaven, you need to first exit from the other side.
Like a real old-school RPG, it starts out super rock-hard, you are weak and it's frustrating as hell, then around level 25 or so it opens up and You Are Gods. Specialize, get grandmaster. Grandmaster anything is ridiculous. I have spells critting for well over 1000 damage.
It's archaic design, and I actually don't agree with it, because many people will quit early on, frustrated. I would have quit myself if I hadn't hex edited in unlimited supplies. And hex editing a savegame is itself super oldschool, I haven't done that for many years. But they said they were making another Might and Magic, and graphics aside, that's exactly what they did.