A few years back my host completely deleted my website and blog. Gone. I ended up having about 2500 404's for about 2 years. They slowing went away when Google realized the situation. My point? Google will get around to it. As far as the URL's to crawl, in my experience, Google will get to them but it will take some time. And with 20, 000, it may be a little bit of time. I'm curious if there is a way to speed up the process, though. Following