A good strategy to minimize backtracking is to level your main job to 18 and get the subjob quest. Then you level your subjob to 37 in one go, since 37/18 is a capped subjob. Then you're set to level your main from 18-75 w/o interruption. Don't do what I did and level jobs piecemeal. It clutters your inventory with lots of extra gear, or you find yourself selling and rebuying equipment much more than necessary.