I myself love the idea of units gaining experience. I once played a game called theocracy where experience was one of the units' stats (along with attack, HP, armor, morale, stamina, you name it!). However, I'm not so keen on the idea that the time a unit is alive determines their experience. Perhaps you could change it to how many kills a unit gets? This would make it so you end up getting "champion-units", who have proven their prestige in battle, kinda like "minor heroes". Just an idea.