That's exactly what happened to me yesterday. I was planning to row 10K, with no particular time goal. I took it easy for the first 2000, and it felt pretty good. And, I wasn't that far off from my 2K PR. So, I decided to push a little bit for the next 2K. That went pretty well as well. So, I kept pushing hard. My third 2000 went pretty well, but I struggled a bit on the 4th. As I started the last 2000, I was hurting, yet I knew I'd have a big PR if I didn't give up.
My first 5K was 20:48 and my second was 20:44, for a time of 41:31.6. My previous best for 2000 was 8:18, and I basically averaged that pace for 10000 meters yesterday.
After I was done, I basically laid on a mat in the gym, sweating and trying to recover. I was on the mat for a long time, with sweat just pouring off of me.
I am planning to get to CrossFit tonight and then do some rowing tomorrow morning. But, I also feel like I might be coming down with a cold, so we'll see how things go.