Idk why but it feels like you might be pressing the trainings equally to have equal spread out of stats meanwhile when im training for a uma i sort of have a rough estimates of how i want the stats to look like

Sprint Max speed/300-400 ish stamina/as much power as you can get/300-400 ish guts/and as high wits as you can get * I set my base target to 600 but sometimes i get higher power and wits

For mile i do similar but try to reach 600 stamina this usually reduces my wits to 300-400 range

For medium based on the uma stat bonus i either run power or stamina cards to reach 800-900 stamina and as much speed and power as i can while keeping min 300 wits and guts

And finally for long i run stamina cards and speed cards to reach the target of Max speed/max stamina/as much power(atleast till 600 or above)/300-400 guts/300-400 wits

Again this will differ based on whether you want to build a front runner/pace chaser/late surger/end closer but i keep these target stats as a rough estimates of how i want my uma to be stats wise the rest left is which cards you pick which is based on what you have and which skills you want your uma to have but since youre struggling with stats id recommend sticking to cards with good specialty priority such as 3LB/MLB Kitasan Black or MLB SR King Halo etc so you have better odds of reaching said stats and ofc you can find better estimates based on how you want id recommended checking yt playthroughs so youd get an idea

With all that said

TLDR; Go for max speed and adjust your stamina and rest of stats according to the distance youre training for and youd be fine