well i'll throw in my couple of penny's . just find a good speed they are comfortable with and let them trot it out .. when is the last time you watched or seen a fox or yote just sprint for the heck of it ? you just want to keep their muscles toned up , your not entering them in the greyhound derby's [ or are you lol ] you keep their muscles toned and their weight in check , and all else will be in place . if they don't have the heart [ or drive ] to finish a race no amount of conditioning will give it to them .
SLOW AND STEADY WINS THE RACE !!!!!!!!!!!!
Yup, a good quick trot and build them up slow till you are putting some real time on the mill with them. It should build their stamina which in turn will build the natural speed at which they work because they will not be tired and should be able to last much longer working. Just a couple of more penny's to add to the pot. lol