Best experience I got to prepare for military flying training came from gliding and flying scholarship.
If you can do a flying scholarship you get the best of both worlds. You get the military selection process and then civvy flying. Though you are assessed by a civilian instructor you gain a PPL which will let you grow your airmanship before going to the military with the onslaught of a very demanding training program where everything you do,say eat and breathe will be continuously critically assessed leaving little latitude for your learning curve.
Don't get me wrong, if you don't have the aptitude you will not graduate anyway. However if you have a head start it helps the transistion until you get to a point where you either stop enjoying it and quit or realize that you are not good enough and quit.
From all of those on my courses who got chopped very, very few said that they felt they were able and were unfairly removed. They just knew that they weren't going to make it.
Good luck, the rewards are great but it comes at great cost.