Depends on level of magic available, too.
Later on, spells like Imprint Souls and Angelic Host can create forces arbitrarily deep into enemy territory. Spells like Faery Trod allow a certain degree of movement to existing armies, and there's always the fun Flying Ship.
Also, later on having a single main army is just asking for being bLasted with flames, leprosy, frost (although Mechanical Men are immune to all three...) or a massive wave of longdead cavalry led by wraith lords.