My dog ran back and forth along the fence. Intransitive or transitive

Transitive requires an object.

Intransitive does not need an object.

Look at your sentence.

My dog ran. Do you understand what that means?

You wrote it, but the meaning is not locked up in your pencil. I am given enough information to know that your dog was on the move.

Ran is intransitive.

Your dog eats cheerios for breakfast which makes him able to run back and forth along the fence. This is transitive. Cheerios is an object. When I was growing up we used to say that objects receive the action. Cheerios receives the eating.