That style of Birks really isn't that comfortable. I have a pair similar, not thong style but with the strap that goes over the top of your foot with a buckle... and have hardly worn them
they are so hard on your feet, just uncomfortable.
However, that said, I do have a pair of Birks which have the soft bouncy inner sole, green leather just over the top of the foot... they are MAGIC! were not cheap... but I have had them now for about 5 summers and Live in them, so I certainly have my moneys worth