Your brake pinch bolt seems to be of the "hole through the bolt" type, and rust or the normal bending and crimping that happens when it's tightened can make the cable quite hard to move. you need some leverage, and fortunately it's there already.

Loosen the nut and tap the bolt back so the cable side is clearly clear of the brake, but still attached. Place some quarters or whatever between the brake shoes and rim to keep the caliper from closing and squeeze the lever. This should give you plenty of leverage to pull the wire back out of the hole. Of course this is the wrong direction, but once it's free you'll have an easier time pulling it through and down farther so you can readjust the brake.

Remember to close the brake QR and set the adjusting barrel to very near it's lowest position so you can use it to tighten the adjusted brake later as the shoes wear.