I went out again on Tuesday night and ran 21.1k in a very respectable 2:11:30ish (my GPS acted up the first few kilometers, so had to estimate. I believe it is accurate to +/- one minute). Just over a week before I ran the distance in 2:35:00.
The time difference was over a minute a kilometer faster. How was this possible? I attribute part of it to talking during the previous run. In addition to slowing us down, I think it was a great workout because breathing is much easier when you no longer are huffing and puffing to speak. Another part is that I knew I could do the distance and my body was that much stronger. A last factor is that once my 10k/15k times were known to be good, I pushed myself that much more to finish strong.
I was very happy with the result. Then I was very stiff for about an hour. I did not think I could eat solid food for about three hours (after drinking a shake and a bunch of water). My body thought it was under attack and I had chills and then fever for about two hours. When the fever was gone, my core felt much hotter than usual until the next morning. I was noticeably sluggish the entire next day but feel great again now.
So clearly I pushed myself a little too far, or maybe just enough - after all I did not vomit (and never have from running) or feel sick for more than a few hours. I also remember feeling this way after a 10k race last year and now I never feel strange after 10k. Hopefully, my body will be more used to this the next time I go out.