When Should Preseason Training Begin?

Our new AAU travel team 12U or Spring is associated with an "academy". we call it a training facility here. the place is one of the sponsors for our team and as a sponsor gives us time at the facility free of cost. we have access to instructors during this time. we work on conditioning, hitting (tee work, soft toss live hitting and machines), pitchers and catchers. We work there as one of our practices. the other practices are done at the field.
We chose the players for the team and are the coaches come game time.
One of the HS teams work out there too. Our kids have been introduced to the HS coaches and the coaches think that what we are doing with the helpof the facility is great. they are glad to see that someone is preparing them for HS ball. Our HC and AC have both coached at the HS level both JV and V. they hand picked the kids for this 12U team.
Currently we play on a 50/70 field. Every third or fourth practice we are going to have them on the 60/90 field to help them get ready for next fall when they become a 13U team.
It wont be so mentally overwhelming.

After watching the 13YO in LL this fall I can see that this will only help. Our 13's this fall were terrible. big or small it was like they forgot how to play once they hit the 60/90 field. Amazing!!