I am playing the game because I enjoy it, and I don't want to end it fast.
What is the fun in ending it fast under 100 turns? You don't benefit from all the technological advances, you empire is under developed.
For me it's not about the challenge of beating the AI, it's about enjoying the medieval times and history which is a very exciting and interesting era. That's why I don't like Shogun(2) TW. In fact I would really like to see a Medieval 3 TW game
On topic: If you have medium quality troops and you are seriously outnumbered by low quality troops (militia included) and you split your army means that your small army will lose more men in the little fights and overall wouldn't you get your ass kicked?
Consider that your enemy has more settlements and more cash so can produce more troops, but for some reason only produces low quality troops.
Can't the AI be modded in such a way that he would fight less battle, with higher quality troops and more decisive ones?