You use there to refer to a place which has already been mentioned.
The next day we drove the 33 miles to Siena (the Villa Arceno is a great place to stay while you are there) for the Palio...
What does the definite article before "33 miles" mean?
I think it means that there are two known points - the departure point (X) and the arrival point (Siena) or - the city (X) and the city Siena. And the speaker is telling that the distance between them is 33 miles
Am I right?
Yes. The speaker is talking about a specific, definite 33 miles of road -- the miles between wherever they started and Siena.