For this type of quilt I would never start in the center -- too many threads to tie off and hide after you are done! What I would do is start in the *middle* of a side edge and work to the other edge, then start in the *middle* of the top edge and work to the bottom edge. That way you have a big X in the quilt adding stability.
Have you layered the quilt yet? If not, I strongly recommend heavily starching the backing before layering. I use a 1:1 solution of Sta-Flo liquid laundry starch and water, "paint" this solution on the backing with a large wall painting brush, wait a few minutes to be sure the fibers have absorbed the starch, toss in dryer, then iron with steam. Starching adds a lot of stability to the backing fabric so it is much less likely to stretch or distort or pucker while you are quilting. I would also spray starch the top to add stability.
Edit: I see that you have already layered the quilt. You can still add stability by laying the quilt out on a large flat sheet and adding several layers of spray starch to each side. (A fan speeds drying between spray starch layers.) This helps even if you are using a walking foot. If you stretch the fabric while sewing, you will find that you will get puckers when you cross a quilting line. Starching helps prevent those puckers where lines cross.