Mate can tell you right now, say you take over Bayern Munichs fitness training, then within a year those players would look shockingly unfit compared to how they are now, because professional sportsman at this level need to be specifically and individually molded, it's long ogne from the days of sinking 12 pints on a friday and getting on the pitch on the saturday afternoon like it used to be.
Doesn't matter how good an athlete or how commited you are, if your left ot your own devices then your fitness will suffer no matter what, theirs a reason that during breaks in the us etc some of those players came back to work at clubs in england to keep their fitness up, also a reason some out of contract players ask clubs if they can use their facilities and staff tp help them keep in shape - think Blomquist for example did that at United after his contract finished
Maybe Barkley is just someone who doesn't fully understand how to get himself in the best shape for playing and has gone overboard on the gym work - that for me looks the case anyways as he has bulked up massively the past 12 months and it certainly isnt fat.
In short he is putting the work in but its not the right type of work to aid him on the pitch mate, its why all clubs have proffesionals these days whose sole job it is is to do this with players