Would roll up half the quilt, start in the middle and work out as you UNROLL the quilt. I would stitch in the ditch and then back-stitch being sure you have locked your stitching, then I just would skip over the tan sashing. Work from the center to the outside. Turn the quilt a quarter turn and repeat the process. In know time, it will be all stitched in the ditch. Trim all the threads on front and back. I have quilted this way many times. You did make such a pretty quilt.