I have been visiting the area for the last 10 years now, as my in-laws live in La Marina, just a few km from EL Hondo. And to date, I never did the official visit on a Saturday morning.
The hides along the Vistabella road (CV-861) are very easily accessed. Assuming you're driving, you can park under a few trees near a bend in the road (https://www.google.co.uk/maps/place/38°09'52.5"N+0°43'47.3"W/@38.1645837,-0.7306642) and cross over a small concrete bridge across the canal, then head right. There are 4 hides along that track between the concrete bridge and the CV-855. Each has quite a different outlook, onto open lakes or reedbeds or marshland.
And there are accessible hides at the San Felipe Neri visitor centre, even when the visitor centre is not open (which is most of the time!). There is a hide behind the visitor centre (good for Marbled Duck), a circular boardwalk with good views over small lakes, two more hides a short walk further on from the boardwalk, and then a tall tower on the path towards Vistabella Road (https://www.google.co.uk/maps/place/38°10'55.6"N+0°46'33.2"W/@38.1821203,-0.7771076)
EDIT: having reread your post, I realise that you probably won't have time to do both the above options. If you're after Marbled Duck, you need to head for the San Felipe visitor centre. The pool behind the visitor centre is usually good for Marbled Duck and Red-knobbed coot (originally, these were re-introduced, with conspicuous collars, but there are now "uncollared" birds seen). Marbled ducks can also be seen on the lake seen from the northernmost (left-hand) hide that you get to by turning left at the end of the boardwalk