I modified the pattern to be worked in the round and to avoid seams.

Would need to go up to at least a size 8 for actual adult-sized feet; these seem smaller than the ones in the picture.

Modified pattern is:
Cast on 50 sts.
Work 7 rounds of garter stitch.
K 24, m1 (insert needle from back to front and knit into front of stitch), K2, m1 (insert needle from front to back and knit into back of stitch), K 24.
Repeat increase row, keeping the increases on either side of the center 2 stitches until there are 86 sts.
Work 14 rounds of garter stitch.
Use kitchener stitch to join the two sides together.

~7 hours