I did a track workout today. I ran 4×400 meters. My training plan said to do them in 1:52. I did the first one in 1:44, the second in 1:42, the third in 1:41 and the last one in 1:39. 1:39 works out to a per mile pace of 6:38. 
I found this neat little calculator on another site, but it was not working. So I fixed it and put it here…
