To get a working military there are two main strategies (plus an exploit)
One is to make a squad of around 10, and set them to train 7/10 EVERY month. Military is military, if they get good their labors all switch off automatically, so they will just spend half a year drunk on your time when they should be getting better.
If your settings are 10/10 and you lack backbacks and flasks they will get unhappy over the long work but they shouldn't die in anycase.
The second is to do the exact same thing is above, only using squads of three with 2/10 set training all the time.
People claim this makes them train up quicker via sparring. I believe it is true but something of an accident---dwarves will only spar with others of their approximate skill level, which means that it is easier for three dwarves to level out and start sparring than for 10 to do so. However in my experience, three veterans and seven newbs in a squad will result in 2 veterans sparring and the third teaching all the newbs into veterans, while the newbs spar amongst themselves.
So small squads and large squads alike train, with the order set for approximately 60-70% of a given squad to be training year round be it one order of 7/10 for a big squad or 2/10 for several small squads.
All melee squads should be set to active/training and provided with a armorstand or weaponrack barracks, ideally by the fort entrance. Equipment of all kinds is optional--they should start training immediately upon the switch to active/training
All archer squads to be set inactive and roundly ignored, while being provided with equipment (crossbow, quiver, backpack, flask, bolts), facilities (a shooting range or two) and ammo--this makes them most effective, ironically enough
the exploit I mentioned in the beginning is the danger room. I used to build large, inefficient single spear ones taking up 3x3 and thought they were spoilery but realistic. Then I built the streamlined, single space, 10 spear, armor stand-behind an internal door-monstorsity and saw legendaries in 10 rl seconds. That is broken. (to be fair, it ended up literally broken. It killed someones beloved pet chicken, and he did not take it well. Since all my soldiers were in that one space of the dangerroom, he took out three men before I even noticed. So exploity?--yes! fun, in the tantrum spiral sense?--also, yes