I know I generally do alot of OFF leash activities to condition my dogs. Swimming is great. Plus hiking, specially when there are hills
I also use a flirt pole, but I get bored before my dogs get that worn out, great for the puppy though.
I DO know that it depends on WHAT you want to condition your dog for. General health is one thing. But when I'm doing agility I need to work on jumping skill and fitness for the dog. If I did carting or weight pulling for competition I'd have a completely different method for conditioning.
Have you seen this? Scootering with dogs:
Sled Dog Central: An Introduction to Dog Scootering