Rio is crazy now with all the new nobles looking for positions and gold, and the old guard DUKE-ing it out. Not the good kind of crazy mind you, but at least there's stuff happening.
Which means regroup in a smaller geography, accept losses and happy hunting.
With these large hordes, Rio should be able to make 2 armies which go bashing, but you'll prob be forced to give up Jidington area, if not the city itself. Look at it from the brightside, you won't be bored for quite a while and you'll be earning lots of honor and prestige
We were doing that even before the invasion officially started, we had 4-5k stacks of undead in like 8 regions before it began.
Come to Riombara for h/p, gold, and drama