It hurts, doesn't it?
I still can't sit in seiza very long, but I am getting better at it. A technique which you might find helpful is to stretch each leg separately first, i.e. sit in seiza only on one leg with the other leg stretched out behind you. On the stretched out leg, do isometric strength exercises by pressing the top of the foot into the mat (rug, floor, etc.)which will strengthen the muscle and tendons on top of the foot and help relax the instep -- hold the isometric for about 10 seconds, repeat as many times as you feel comfortable, then change legs. Do this three or four times, take a break -- and then sit in seiza. Time yourself and see if you don't improve. Time is the key, however. Don't expect rapid improvement, but do expect improvement. I practice this technique, and it seems to work for me.