John, I don't use any resistance with the dog on the jenny. Just getting that out of the way.

All of my dogs, from the time they're pups, get to play with Kong toys. I have a flirtpole with a kong as the bait, and I simply start flirtpoling the pups as soon as they're old enough to run around and chase things. So, from that time on, all activity involves the kong. So, on my jenny, the kong is attached to a rope. The rope is long enough that as the dog runs, the Kong bounces all willy nilly in front of the dog, and they lose their minds chasing it.

The dogs give a lot more effort on the jenny than they ever have on the mill. They're on the ground for starters, so that eliminates them being elevated at all. The jenny makes no noise, so you can eliminate that aspect also. The faster they run the jenny, the more out of control the kong bounces. which in turn, makes them run it at a faster pace.