Compared to today's soldiers, 19th Century and prior soldiers were incredibly tough!!
First, consider tough training in a modern army: During one of the nights I spent in Ranger School, towards the end of a continuous patrol that lasted 12 days on about one C-ration per man per day and maybe 4 hours of sleep (in 30 minute snatches) per day, we moved about 8 miles through the mangrove swamps of Eglin Air Force Base (Florida) carrying our standard 50 lbs of gear plus weapons, radios, etc (everybody had at least 65 lbs, and many had close to 100 lbs), carried out an ambush, and then did 5 miles at double-time. For modern soldiers that's pretty tough training - of the 450 hand-picked ROTC and West Point cadets who started that Ranger class in 1977, only 175 of us graduated. I was a wiry bicycle racer who was used to riding up to 100 miles at a time in those days, but I still lost about 40 pounds during Ranger School.
Then consider Davout's Corps at the Battle of Austerlitz during the Napoleonic Wars: They marched close to 80 miles in roughly 24 hours, and after arriving on the battlefield early in the morning, they got thrown into the toughest fighting of the battle, which lasted for an entire day. Physical demands like that, combined with poor wartime food, sanitary conditions, and medical care are a big part of the reason that disease and injuries caused more casualties in every war that the US fought prior to Viet Nam.
Casualty rates and the causes of casualties during the American Civil War provide another example of just how brutal wartime conditions must have been: Prior to their military service, most Civil War soldiers had been tough enough to survive their childhood and teenage years under the incredible physical workloads, poor diet and sanitary conditions, and practically nonexistent medical care of 19th Century farm life - which would literally kill 99% of modern Americans in under a month. Nevertheless, Civil War regiments generally had about 900 men when they started training, but by the time a typical regiment saw its first combat, disease and injuries had already reduced its strength to about 450 men. And after a full year of combat, many regiments were down to 25 men, with disease and non-combat injuries being responsible for most of the losses.
(Plus, you can reach out, if needed.)
