How people define vegetarian is often different so a clarification may be helpful. Are you looking for strict vegetarian (vegan) food or, as many in North America do who call themselves Vegetarians do, are you looking for places with fish and eggs as well as vegan food (I've come to accept this distinction but have never understood it )!
If it is the latter you will have no challenges at at and good variety.
If it's the former your choices will be far fewer but not non-existent. There are usually great soups (but may be made with non-vegan ingredients like chicken or beef broth so ask first if that's important to you), some salads, pasta dishes that can be non-meat adapted, always French fries and lots of fruit at Tiendas along the way. It may not be fancy but you will be able to find what you need to survive! Buen Camino, Jordon.
ovo-lacto (sp?) vegetarian - Doesn't eat any meat but does eat egg (products) and mil (products)
vegetarian - doesn't eat any animal products like fish or meat or sea food
vegan - doesn't eat nor use any products that proceeded from an animal, including leather, honey and the like
To the OP:
Your best bet, if not speaking Spanish, is preparing a laminated flash card stating your dietary preferences and showing it to the waiter/shop keeper. In your case (assuming ovo-lacto vegetarian) that could read:
Soy vegetariano/a, no como carne, ni panceta, ni marisco, ni pescado. Me puede recomendar algo?
I am a vegetarian. I don't eat meat nor bacon, nor seafood, not fish. Can you recommend some dish for me.
Buen Camino, SY
PS There are a 'few' Spanish vegetarians around also, so it isn't an out of this world problem ...
Most 'menu peregrino's offer mixed salad (insalata mixta) which is fine for veggies/vegans as long as you specify 'no tuna' (sin atun). Even if you say you are vegetarian and you don't eat meat or fish, you'll still be served tuna as part of your salad!