Good Afternoon ! The hotel is 18 miles to the Riverwalk or around 20 minutes drive time.
You can probably get to downtown and parked close to the the Riverwalk in about 20 - 30 min. There are many little lots you can go to, but the River Center mall parking garage is my favorite place to park if I go to the Riverwalk or the Alamo.
Get an answer from Courtyard San Antonio SeaWorldR/Westover Hills staff and past guests.