For game shape you probably want to target 40 to 90 minutes. BB-Forest at one time indicated that the drop off wasn't that severe just outside. You might have to accept not getting as much training as normal, or maybe training stamina or FT.
If I had a starter at 70 minutes, I'd probably let him start if I wanted to win the cup game. You're unlucky to draw another D.III team. There are lots of bots in D.V and if you win your first game, your game next week might be one that you can use some trainees mixed with better players.