Greetings from Jerusalem!
A Summer Camino will be hot very hot-30+ degrees easy bring your favorite container for your preferred beverage-a big container. 18 days is not enough to do it all so choose those sections which are totally different from where you live. Which means, if you live in someplace like maybe Kansas don't walk the mesta it will be just like home do walk the mountainous stretches which will be different-if you are Swiss do walk there for the same reason. If you live in a "classic" European city you may not get excited over the Medieval or the Renaissance art and architecture you will see in Pamplona, Estella, Burgos, Leon etc – if you live in America don't miss any one of them. And lastly, don't make a schedule, walking slower than your timetable, stopping sooner than your prepared itinerary, visiting a church or a monastery not in the guidebook together with new found friends and companions who are sharing your own real Camino is far more memorable, moving, and important.