Use "not that ___ of a ___" when you want to:

• describe what something is not ("long")
• tell what category it's in ("a hike")

For example:

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.