Cost of Senior card is 6 Euro. It gives 40% discounts on selected days.
Cost of ticket on this Wednesday's 1440 train is 38.40 Euro. With 40% discount cost of ticket is 23.04 Euro. Net saving is 9 Euro.
You should be able to buy Senior discount card and Renfe tickets at the airport Cercanias station or the Chamartin Station prior to boarding your train.
Information about Senior's discount card is here.
http://www.renfe.com/EN/viajeros/tarifas/tarjeta_dorada.html
I wouldn't normally consider purchase of tickets in advance for onward travel on a day arriving by plane. Too often there are significant flight delays. You have lots of time though and buying in advance may result in significant savings.
You could try purchasing Senior Discount Card and tickets from Renfe Sales point in Canada.
Renfe's International Sales Points
http://www.renfe.com/viajeros/viajes_internacionales/puntosventa_internacional.html