I'm new also with the game, but I try to do all the training the game asks from me . And , of course, after you made those sessions ( they don't need to be made all at once ) you can extra train players for the future ( to grow them ) . For example, with all the training asked for me from the assistant and with only 2-3 extra training sessions made from my head, I had 2 players that grew in the last week and a half, one was 24 and one was 20 , but that is random because their stats were peaking .
Cheers .