I understand that it is just a dynamic of gameplay but one funny thing I noticed in my full English campaign in MTW was that I had standing armies that stayed in one spot literally for hundreds of years.

I had an "alliance" with both Poland and Hungary who stood as neutral ground between myself and the Byzantine empire. As we all know alliances aren't worth diddly so you had to keep defensive armies on each province to deter an invasion by a neighboring nation. So I assigned archers, fyrdman and urban militia to stand in these armies and they stayed there for doing nothing for nearly my entire game until I decided to go head on against the Byzantine. Kinda ridiculous, I paid these armies for hundreds of years and they stayed there at the ready all this time, hehe... they must have been an undead, resurrected army for a good part of that time.