itunes 11 podcast "episodes to keep" setting

Hi,

I have subscribed a podcasts that releases new episodes very quickly, and now I have several hundreds of unplayed episodes. I edited the podcast setting to "keep last 10 episodes". I clicked "OK" and then refresh the podcast list, but nothing happened. Do I have to manually delete those "excessive" unplayed episodes?

Also, what should I do if I want to keep the last 20 episodes? (iTunes does not have this option...)

Thanks.