I noticed the dog 'treadmill'---thought it comparable to a 'hot walker' for horses (think that's what it's called)

I have no doubt that the dogs must be in tip-top shape to run....

I think that the word 'cull' has bad connotations--most people instantly think kill (myself included, even though I know there are other ways to cull) Maybe a 'career change' is a better term