R2L, your dog could've just as easily twisted something slightly on the mill. There's no guarantee that 30 minutes is the cause of it. A dog should be able to run for 30 minutes on a mill after 3 weeks without any type of issues given they're in relatively decent shape to begin with.

A lot of keeps are opinionated. Take OG. He uses a collar. I use a harness in all work I do whether it's mill or a jenny. I don't think 30 minutes of sprinting a dog is peaking a dog as I have my dogs sprinting from the first week to the last. It's all opinionated on what works for someone or what they like for their dog to do. Did you check the dog's pads to see if they had any damage from the running? I personally don't see how a harness is going to make a dog limp simply because a harness was used. If the dog is limping, it's from doing something physically not from wearing a harness.

My personal thoughts are that if a dog can't give you a good 30 minutes after 3 weeks, then you may be working the wrong dog.