To clarify. befor embark. I use dwarf therapist to look at all of my dwarfs to see who has the best potential to be a soldier. I especially look for ones with good kinesthetic sense and avoid ones who are clumsy and weak. I give them +5 fighter and +5 axe skill on the embark screen. As soon as you arrive, build a carpenter workshop, dig out a hobbit hole, craft a armor stand and make a barracks in there. craft some training axes and wooden shields and assemble your 2 man squad. Since their skills are equal to each other, they will spar 90% of the time. Military skill growth is exponential. While it may take a year to get to rank 5 fighter, it also takes 1 about year to get to max rank 15+5 fighter. the better the skill, the more attacks/hits they have to throw per sparring session. You can see how it would grow super fast. Fighter skill is the most important skill for survivability next to shield user. It lets them attack faster, parry, counter strike, etc.
You just want them to build up their core skills.
Fighter
Shield User
Axedwarf
For this, all they need is a training axe and a wood shield and a place to train.
Just use 2 soldiers per barracks so they will spar more often.
Green, fresh recruits take longer. It usually takes them 2 years to become legendary, maybe longer if they have extra skills like wrestling, dodging, etc. They will waste time doing wrestling demonstrations instead of sparring. Theres nothing you can do, just let them. Just know they will take longer to train.