Since currently leadership only levels when leading armies, and leading armies is only possible during the late-game, upgrading troops should train leadership. This makes sense since leadership is the governing trait for how quickly you can train your troops.
Frankly, you should be getting leadership XP constantly as long as you have troops to lead; the bigger your party the faster.
Frankly, you should be getting leadership XP constantly as long as you have troops to lead; the bigger your party the faster.