I grabbed this image from the website of Redline rentals which is the place that rented us our first Kayak. Try them out if you are new to the area. They are very close to Granite reef where my wife and I first hit the river. A day or so later we had purchased our first kayak that we found on Facebook marketplace.
After we drop the pick up car, we head upriver with the Kayak on the roof to Water Users Recreation Area. Once there, we use the most eastern access ramp down to the river to launch. There are a couple of western launch sites but we prefer to launch from the lagoon and get the absolute maximum out of the river. Using Google Earth I measured the entire route and it’s 12 miles give or take a few feet from put-in to take-out.