“It's not that long of a hike.”

You and a friend are in the countryside. You want to hike to the top of a small mountain, but your friend isn't sure if he wants to do it. You're trying to convince him to hike up there with you. You say:

It's not that long of a hike.

not that (adjective) of a (something)

Use this construction 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 the examples above put a little more emphasis on the category.

a hike

"Hiking" means walking over rough ground, like in the mountains, in a desert, through a forest, etc. People usually hike for fun.

"A hike" means one complete session of hiking. This can last for a few minutes or for several days:

We did a ten-day hike along the Appalachian trail last summer. It was gorgeous.