I think there are two different questions in this thread. One is how to get out of the box that Turkey's initially contained in, and get into a strong midgame position, while the other is how to reach 18. The second one is, I think, the more central question, because it relates to one of the most important skills in Diplomacy, taking a board and thinking about what happens when it shakes out, who's going to end up where in several years.
Basically, from the moment that it's clear Turkey is going to be growing, there should be one thought foremost in your mind: you cannot win without, at a minimum, at least one of St. Petersburg (which you cannot hold long-term), Munich (which you can), Berlin (which you can but either with Munich or Prussia and Livonia), and Marseilles (which you can). For many countries, you can push on all fronts, hoping to succeed on one. For Turkey, because of the long lines of resupply combined with the tactical ease of assembling a multi-national line against you, even in gunboat, you can't -- you have to pick where you're trying to cross, commit to it early, and pick right.
As an example of what I mean, take a look at
gameID=87888. As early as Fall 1906, I needed to think seriously about which I was aiming for, and concluded that France might get much of Italy, so that I was basically racing for Scandinavia or Munich. In 1907, at that point I had to basically make the decision and commit, because if I waited it was a clear EFT three-way draw. My conclusion was that my only chance to win was to lose ground against France and aim for Munich/Berlin, letting the tactics be messy, because the alternative was dead drawn. I did get Munich/Berlin, the tactics did get incredibly messy, but it was the only play that wasn't dead drawn, and ended up a very complicated, difficult win that was actually complex but forced about six years before it ended (it would have actually been easier without the French NMR that let him retreat into my rear, and at one point it was actually a second-order virtual win, if I analyzed it correctly).
Basically, the point is that as Turkey, you have to make your decision earlier than everybody else about where you're getting across the line, commit to it, and get it right.