This one has stumped me. Here is my sentence in question:

A forty-second walk on the beach, a few steps up a very small hill, fifteen feet down a trail, and we would be there; piece of cake.

Forty-second is an adjective that modifies walk, so I hyphenate it.

I cannot figure out whether or not to hyphenate fifteen feet. Fifteen feet is not an adjective, but what is its part of speech in my sentence?

I would say, “Do not use that fifteen-foot board there.” Or, “That board is fifteen feet long.” But, I do not know what to do with this sentence.

Do I only hyphenate in this instance when a compound word using numbers is an adjective?