We have been lucky so far in not needing to use the Spanish national health system (touch wood!) however many of our friends of a similar age have not been so fortunate. The good news though is that their experiences have tended to be very positive and they all say they wouldn't have received such good treatment if they had been back in the UK.
My general advice is to adopt the Spanish way of eating and drinking e.g. if you're out in the evening for a few drinks with friends, always have tapas with your drinks. When you are having wine with a meal, also ask for "agua", especially in the summer months. Make the most of your local market, with lots of fresh fruit and vegetables, plus go to your local pescadería and carnicería for fresh fish and meat. We have noticed that Spanish supermarkets are stocking more ready meals than when we first moved here, but on the whole Spanish families still prepare most meals from scratch.
Try and get some exercise every day. We see groups of Spanish women (and men) out for walks in the morning, wearing trainers rather than their usual high heels (the women), so clearly taking it fairly seriously - though not walking so fast that they can't keep up a conversation, judging by the noise!
Sue