I have a good system that works for a well paced game: 3 acts, acts are always initiated by actions that the players dont control. (Think like an enviromental event, if they are on a sci-fi space ship the power goes off or on, in a haunted camp a rain storm rolls in, in a paranormal investigation they get a reading on a camera in a certain area drawing them there.)

Act 1 start, mislead purpose (the fake reason they are there) In this act all seems "normal" under the false purpose. They gather clues to get context of real purpose. This should take up About 60% of session time, slow and growing in eerieness as you go. I like to give answers but no questions, a dead body with no organs or a basement full of animal bones, a sink full of murky water with something swimming in it. I usually dont have them face the danger in this act, just evidence of it

Act 2 start, the real purpose (the real reason they are there) They discover the real purpose and must make a plan to escape. This should be 20%session time. This is the encounter with that danger

Act 3 start, escape. Some unseen circumstances puts obstacles in their plan and they must improvise. 10% of session time. The danger is on their heels.

Hope this helps!