Use "not that ___ of a ___" when you want to:
- describe what something is not ("long")
- tell what category it's in ("a hike")
She's not that good of a singer.
This isn't that old of a building.
It really isn't that big of a deal.
Of course, you could also say:
It's not a very long hike.
She's not a very good singer.
But these versions put more emphasis on the description ("long", "good"), while "not that ___ of a ___" puts a little more emphasis on the category.