I have mate like you say a bull and a grey are athletes but believe me they got not much more than a pure grey running wise all the bull does is stop them been to weak
a greyhound you couldn’t run a full night on lamp with you’d have to carry it home there to muscle bound for short rapid bursts soon burn out
same with a bull terrier muscle bound not made for running long distances fighting pit on pit is using the muscles in a different way
no amount a care and fitness sort that out