It doesn't hurt right away, actually. It starts to hurt after a few minutes, then the pain builds up until I can't even walk normally and have to either kick off the shoes or limp around.
So it turns out that when you cut up a leather jacket, the actual leather is extremely thin. The thickness and heaviness of these things are because there are two layers of leather with some insulation in between them. To get a proper barefoot feel (and to avoid hours of sewing), I'm going with just one layer for now. Actually, the paracord is turning out to be overkill for something so light and thin. I tried traditional tying and it was definitely better than with the rubber ones, but still after a few minutes it started to hurt. So I'm going to move the holes a bit so the sandals are super-loose, and replace the paracord with an old light shoelace or long strip of leather. The sandals are so light, it shouldn't do much harm - I don't need much force to keep them on.
On the other hand, because they're so light and thin, they tend to flop around where they're not attached to my feet. I'm considering two ways to deal with this. The first would be traditional tying PLUS an extra lace that just goes over my toes to keep the leather from flapping around. The other is to tie them in a different way, criss-crossing over my feet so they're attached in several places at the front and back.
As requested, here are a few pics:
My workstation with abandoned rubber:
The leather jacket:
Completed first try at leather sandals:
Bonus shot of my cat sleeping on the remains of the leather jacket:
EDIT: Success! I have found a way to tie them that works great and is comfortable. I took a lot of pictures while tying them, and I'll post them soon with an explanation of how to make these in case anyone else wants to try. It's actually very easy and EXTREMELY cheap, not to mention far healthier for your feet than anything you might buy in the store. All you need is something leather that you can cut up (easy to find something at a goodwill or secondhand store) and two pairs of shoelaces, as well as scissors and something to poke holes with. And I actually find that tying the laces is really fun. I've done it so many times now with all my failures that I'm a real expert! But now that they're done they'll slip on and off, so I won't need to do it again unless a lace breaks (not likely, since they don't touch the ground like they do with the traditional between-the-toe lacing).