Gotto agree with stabba, NEF and few others. Everytime bullxs and fields being driven are mentioned you all kick off. Again skycat and whin are stirring and again I bet neither have ever run a decent bullx, walking or from the motor.
You mention stamina whin, now if your driving fields I think dogs need more stamina as they have less time between each run. Walking you may not get a run for 2 fields, this may take 10 minutes whereas driving it may only take 1 minute.
As of the flat feet; do all danes, deerhounds, wolfhound and large greyhounds have flat feet?