I've had my initial trained units survive until I could replace them with mounted units at which point I use them for garrison duty. The first mounted units I build often survive to the end of the game. I equip them with ranged magic weapons and use them as artillery - it's the only way I've discovered to take down more powerful monsters in the early game. I upgrade them so they are still relevant in the endgame - a level 12 company armed with hailstone or hellfire staffs can inflict a lot of damage.

Hmm. My trained unit survival rate is really low, this almost never happens.