How would one say "He runs the length of the street"?
"הוא רץ את אורך הרחוב"
Very good deduction, friend. אורך is "length."
Is there a different way to say, "He runs down the street"?
הוא רץ במורד הרחוב
Why is this requiring a past tense form? Do we " know" that yet?
What past tense? רץ is both present tense and past tense.
No. If it doesn't have houses, it's not a רחוב, but a דרך. So רחוב is like a street, and דרך is like road.