Weekly Improvements?

Last week I ran my 2 x 2 and really struggled to maintain an 8:45 pace through that run. Yesterday I was scheduled for 2 x 3 with the same goal pace. I nailed it, it was easier than last week, I even went a little faster and 2 additional miles. Holy crap, really?!?!? I was shocked, didn't really believe it at first. Here I am moving along quite well into mile 3 of the first set and I'm surprised to notice that my heart rate is down and I feel strong. I do my 5 min recovery... stop to pick a rock out of my shoe... and then I'm back moving again for the second set of 3 miles. I knock it out with a faster pace than the first three and practically a lower heart rate. I felt GREAT! All the crappy runs I've had this year, they are completely forgotten when I go out and have an awesome run like this. I am so excited about my 10k next week and I have complete faith that I will hit my 8:45 goal pace, and maybe even a tad bit faster!