Which word is being used as a preposition in this sentence? the dog raced toward the lake, too.

The answer is:  "toward" .

Explanation:
 
The answer is: "toward"; since:

       1) "toward" is the only preposition in the sentence given:

           "The dog raced toward the lake, too."  ;  AND:

       2)  In the sentence given, the word, "toward" ; is, in fact, being used as a preposition.

To create unity throughout the poem