The avid runner ran slowly and steady until he crossed the large finish line.

Explanation

The adjective "steady" should be changed to the adverb "steadily" to correctly modify the verb "ran." This option had a parallelism mistake, parallelism is the repetition of grammatical elements in a piece of writing to create a harmonious effect which means when providing a list of items within a sentence, mention all of them in the same syntactic form so when slowly is used grammatically steadily should be used instead of steady. Moreover, many adverbs in the English language end with the suffix -ly, so "steadily" is the adverb form.