I did some treadmill training with one of my Great Danes as well as my Bengal cat. We got to the point where they'd be on it for a minute or two, but then I sort of gave up on it. I just took it slow and used positive training methods and it wasn't too hard. I think if I had been more motivated to stick with it, I would've had a dog and cat who could run on the treadmill
I used an actual human treadmill, both because Danes don't fit on those little dog ones, and because if you're going to drop the $$$ on a treadmill, might as well get one the humans can use too.