I've been reading more than usual about our Civil War back in the day and remain impressed with several basic things:
1. How armies back then were not wiped out by disease, and of course more of the 620,000 or so who died did so of disease than combat.
2. How they could at times line up 60 yards apart with rifles and have at it, as they did at Second Manassas for an hour or so, in the open, and not wipe each other out.
3. How they could march so far in a day, and get up and do it again, often barefoot (Valley Campaign 1862).
4. How they appear relatively impervious to heat, cold, mosquitos, ticks, fleas, insects of all kinds, snakes, and lions.
5. How anyone could effectively control an army of 60,000 individuals. I know in 1862, that control was often very limited.