Constantly dealing with 4+ enemy armies before turn 10 on normal mode?

I just bought the game and I enjoy the RTS battles, but I'm pretty confused about how to defend my territory in campaign mode. I've tried two games on Immortal Empires with default settings, one Empire and one Cathay. Each time my territory was invaded by 4+ different factions. Even though I could beat their armies, I literally couldn't be in enough places at once, since any settlement I wasn't defending with a full army would get immediately sacked. Additionally, I couldn't deal with armies in my own territory, since they were always invisible, faster than me, and for some reason literally ran away during my own turn once I caught up to them? It seems like the only way I could force a fight was when I was besieging one of my own captured settlements. Does anyone have any advice?