When I went 2 years ago, I knew absolutely no Spanish nor French, assuming everyone would speak at least some English nowadays. Turned out the moment I got there, I found out I was absolutely wrong. the thing is, though, I survived. Yes, it was hard work to communicate to get credential (I started at SJPdP, too), lodging, food, anything. But anyone can tell you are a pilgrim when you are at SJPdP with a big backpack. I actually met 5 other pilgrims on the train to get there, some whom I got help from with first day lodging.
After I was on my own, I drew pictures when I didn't know Spanish words for sandwitch, apple, or any other food, or even toilet. There were always people who could translate, or someone who want to learn and talk in English. and those who only spoke Spanish (and locals) are used to pilgrims who don't know anything beyond 'hola.' I found people generally are so patient, they acutally wait for you to find a word you are looking for in the dictionary. Besides, you will learn the most basic stuff on the road, and actually get used to Spanish, for sure.
Don't get me wrong, it's great if you have time to learn some Spanish before getting there, and I highly recommend it. But not knowing Spanish does not kill you. As the other guys said, SMILE, have fun, and just try hard. That's what I did, and it worked well. And don't forget a mini-dictionary.