I describe my own experience as I went on the Camino de Santiago in summer 2011.
I will focus on Spain and the camino frances, if you want to know something about france, switzerland or germany I will be more than happy to add my experience in these countries.
To answer your question: yes it is absolutly realistic. But maybe you need some time. I met people in the age of 60 and much older and they have been fine on the way.
Your questions addresses two parts: infrastructure/supply and your personal endurance.
In Spain you will find a very good infrastructure for pilgrims. Most of the villages seems only to exists because of the pilgrims. You may find one to more pilgrim hostels and classic hostels and hotels (I recommand the pilgrim hostels for the most pilgrim experience. You are always able to eat in a restraurent, which may be expensive and some beautiful hostel wardens conjure beautiful dinners. Most of the time you are also able to cook by yourself and I think thats the way most people do. Even the small vilages have of course the necessary food offered but this is usually more expensive than in your discount market. If you think, these information are five years old, you're right. But you may assume, that the infrastucture went better and not worse because of the popularity of the camino.
Most of the time, I have enough food to cook for the current and the next day. In your guide (miam miam dodo, don't buy it in advance but in the first hostel.) you will always be perfectly informed about the next stops and the infrastructure.
Most of the people are non trained and most of them have just bought their equipment right before the trip. This is not an advice, but something I have observed.
The really hard part in spain is your first day if you start in Saint-Jean-Pied-de-Port. The part to Roncesvalles is very steep for non trained people: just go slowly, hold on your own pace and make a lot of breaks.
Generally, if you not used to walk (even not in the past) take a lot breaks! Just start with 20-22 km per day and you will rapidly see an improvement of your endurance. If you are older or weaker you may start with 10-15 kilometres per day.
Do not forgot your own weight and the one of your bagpack: My weight was around 75 kg in this time and my bagpack was 9 kilos without any supply. Add 1-2 kilogs for water and 1 kilo for food. And maybe the half package of nudels from yesterday. Having an bagpack from one day to the other on your shoulders mean your body have to carry about 10-25% more weight than the day before! This means a lot of stress for your muscles and maybe bones. For visualization: Imagine how many burgers you have to eat for the same effect - in a very short time. And while these 9 kgs for your own things may be constant, your weight isn't and you may weight less than me.
The second "hard" part you may encounter is the "dessert part" called Meseta between Burgos and Leon. This part is very flat and you will see less fauna than the other parts. You may have longer distances between villages than on other places. After this and with the sufficient training on the trip you have built enough muslces for the other parts.
Long story short: go for it. Take your time. If you are absolutly unexperienced and un-trained, I hope you have 6-8 weeks for the spain part. So you do not have to go fast and enjoy this very special, unique journey which does not only happen outside you.