US Navy SEALs have a saying, "The only easy day was yesterday". The military has a long history of preparing its soldiers for combat readiness with physically and mentally demanding training regimes. The idea is that men and women of the military will learn and be conditioned to cope and, more than that - excel, under extreme physical and mental duress.
Comparisons between the military and sports training are symbolic rather than real (more on that in a future posting). Fitness is specific to what you want to be fit for. The elite marathon runner would likely excel over 26 miles and 385 yards against most SEALS. But does that make either fitter than the other? Clearly not.
As I noted in an earlier post, the Crossfit claims of - the "world's fittest athletes" does not answer the question, "fit for what?"!
Training regimes must be designed to meet the physical, mental, technical and tactical demands of the sport. Sports with a high physical conditioning component will have more of the SEAL-type challenges to operating under conditions of extreme fatigue and oxygen debt. Those with a high level of technique and little speed-strength and endurance challenges - won't.
But then again, training hour after hour, day after day, month after month and year after year, is the world of elite performers in any discipline. Ask a concert pianist, violinist etc; or a ballet dancer. They too will tell you that "the only easy day was yesterday"
(photo source: Alvin Ailey American Dance Theatre)