Lot of people missing the part where the OP doesn't want an app to get him to a particular location, but rather an app that let's him follow a predetermined route.

What I use is MotionX GPS. I do not know if or how it can be made to do turn by turn, which is something I would really like as well. What I can do is create a route on Google Maps by putting in a start and end location and then dragging the route around until it goes where I want. Then I use the instructions on this page to get the Google Map route into MotionX.

In MotionX it does not show up as a route, but as a series of waypoints that mark your turns. If you're in the habit of checking your screen regularly, and your battery holds out, that's sufficient to keep you on track. It also works if you just want to double check and make sure you're taking the right turn.

What I'd like, and what it sounds like you would like as well, is something that would prompt you at the turns to remove some of the necessary vigilance when looking for the next turn. I keep thinking that MotionX is almost there, but I can't find a way to get that functionality out of it.

Also, when using MotionX, it's good to keep all your routes in separate folders in the app, keep them backed up, and delete them from the app as necessary. Because it's just a bunch of waypoints, overlapping routes can be confusing.