We arrived at 11 and booked the tour from a little booth situated near the visitor center. We took the 2h tour.
We drove inside the park up to the stable and waited an hour for appropriate horses to be available. It is the only moment when we were a little worried because the guy there seemed quite unsure about how long it would take.
Everything afterwards was wonderfull : the guide named Billy, the horses, the lanscape, the pteroglyphs, the numerous explanations, the gallops...
Many thanks to Billy !