Sorry - the St st (stockinette stitch) is knitting all the way across and when you turn the work, you purl all the way across. So, yes - the St st is in the middle of the garter st border. Check out the videos on this site to help you.
You could also just knit an entire afghan in garter st (knitting every row).
Knitting/Crochet/Crafting/and Related Product Review Blog: KnitPurlGurl
"Yarn stashes insulate the house."
"Just one more skein can't hurt."
- from Knitticisms