You shouldn't really use Personal Trainer it's a rip off. If you've got tokens to spare and want to use them improving players it's much better to buy green packs. Train players on hard setting use greens to recover them and repeat again and again

You may need some reds as training like this obviously leads to injuries happening more often

To be honest if your only on level 5 i'm not sure it's worth doing mainly for these 2 reasons

1. You can only do this sort of training on young high value players, i don't do it on anyone over 21 but some people on here might do it on players a little bit older.
2. I bought and sold a lot of players early on in the game so its probably not worth starting to do intensive training on players till about level 7. Again some people might disagree but early on you struggle to find really high value 18 yr olds who you know will be worth it there's a lot of trial and error in finding the really fast training players.

Hope that helped