For example the Romans. About everything we've seen about them has shown them really really muscular. How did they do it? How were they able to consume enough calories and how did they train to look that well? Also how did they maintain that muscle? While away during war I doubt they had enough rations to hit the calorie limit of 1000's of men.
I know that Roman soldiers trained by building fortifications and going on long, forced marches. They also, apparently, used weighted swords in practice.
If you want a wonderful, in-depth analysis of this, read Caesar or Hannibal, both by Theodore Ayrault Dodge. Of all the historians I have read, he seems to always go the furthest in explaining the practical sides of war. He endlessly describes formations, training, logistics, ect.