Yes a dog can be made better by being fitter, heart comes in when there tired. I'd disagree with that totally. Heart (gameness) doesn't come in to it with fitness. I've seen terriers that are fit as a fiddle walk after 5 minutes and refuse to enter without even being out of breath.Obviously day to day fitness such as regular walks and exercise are necessary but optimum fitness found on a treadmill won't make a dog that lacks heart any better IMO.
Seen terriers quit seen lurchers stop chasing seen bull dogs stop because they didn't want it anymore, nowt you can put in if the heart isn't th