Courses can provide turn by turn directions but they must be manually added to the file. Some of the websites that create courses will add them for you however I find that using Garmin Training Center allows me to put in the ones I want and exactly where I want them.

Another major drawback of using an iPhone to do all of this is battery life. I am guessing if you keep the screen on the entire ride you may get at most two hours before the battery dies. With it off up to four hours.