Large army practice

Is there a custom scenario which focuses on training how to manage large armies and late game techs? Im new to the game but i already have a pretty solid foundation on the early game, but when i reach the mid-late game i just get overwhelmed by the amount of variables this game has. I can win a game if i do a good rush, but if it extends to much i almost always lose. Im asking for a scenario because the art of war challenges and the zeroempires scenarios are what really helped me improve, and they are basically the reason i didn’t give up on the game at first.