When you start, stitch from top to bottom at your center seam. Then stitch from side to side at the center seam. This will secure the layers. Then, go from top to bottom alternating on the left and right sides of your center stitching.
Rather than worrying about staying in the ditch, you might think about doing a slightly curved line going from the top of the block to the bottom and weaving from side to side of the seam.