If you have the €23,50 per person seat reservation on AVE trains, a meal is included. It will be written on your pdf ticket as well somewhere (desayuno/merienda/cena/...). You'll get served whatever they have left.
Renfe offers the option to pre-select from a menu, as part of the booking process when booking regular tickets. You could try to reach out to Renfe customer support or to one of their ticket offices and ask if they can make that selection for the reservations you already have. They always have at least a meal and usually minimally two to chose from on board if you haven't chosen before. In fact, when I did this (with a regular Premium fare) the one I had chosen wasn't available but that was when this service was just introduced.
Renfe also loads specific meals for dietary preferences. Those do need to be booked ahead, as otherwise they are not available - reach out to Renfe through the phone. In my experiences that service (which they've offered for longer) has usually worked.