Are you planning on using just straight lines, stitching in the ditch? If so, you can fix this. I don't know how to give advice for anything else, so that is what I am referring to....just straight lines, like a goal of so many straight lines from one direction and then others perpendicular to that... I use Warm and Natural, which you can quilt as far as ten inches apart...check the instructions on your batting to see what you need to do. I rarely stitch as far as ten inches apart though...
If your first two seams are okay, you could leave those, but pick out the third seam. (If you have pinned about every 4 inches, you need to do that...but I am assuming you did that since you did a lot of research first.)
Then DO start from the center, sort of. Start from the center of one of the lines. Sew all the way to the end in one direction, and then go back and sew all the way to the end in the other direction. Do that again a couple of times, smoothing out as you sew if you need to.
Continue sewing straight lines out towards the ends...basically sewing from the center as you suggested. (do perpendicular lines too...so you form boxes)
This isn't very clear, is it? But the idea is to sew out from the center, as you suggested. I used to do this all the time until I got used to the quilting process more. I stitch in the ditch though, and that is all I do. I don't stitch in all the ditches though, just enough to meet the requirements of the batting I use.
Good luck. This shouldn't be real stressful. It should be rewarding. It may not be fun, but it sure is cheaper to do it yourself than pay a long arm quilter.... You might want to review some of the research you made before you started too.
Dina