On my trips to Spain in 1995 and 2006, I found very little English (and this was in touristed cities like Madrid, Barcelona, Seville, Cordoba, and Granada). However, on my September 2015 trip, I definitely found more English in Madrid and Barcelona than I had on prior trips.
I don't know if I was doing something different or if there really was more English, but I do feel that in the places you want to go, you won't have much trouble with only English. Of course, the more Spanish you can speak, the better. It's not hard to learn basics (please, thank you, hello, goodbye, where's the toilet?, and numbers from 1-10 are a great start). Get one of those "Spanish for Travelers" CD's (try your library first), but make sure it's for Spain and not Latin America.
One definite issue is that the menu del dia (the set menu of the day, served at lunch) is not only a great bargain (€10-12 for 3-4 courses), but also has more variety than some other food choices (which, if you're careful, can seem to consist of nothing but ham and sea creatures). However, since the menu del dia changes daily, it is usually handwritten in Spanish only. Bringing a phrasebook to translate it may help.