I ran 6 x 1000 today at 3:38/interval. I nailed the paces, even though I stopped the first interval after 600 meters. So I lost a few seconds, and when I realized the mistake, I started to sprint again, finishing the interval on time. Maybe this is the reason why by the 5th and 6th, they felt very hard - indeed, this was my hardest workout in a long time. I had to seriously sprint the final 200 meters of the last interval to get in on time, and it hurt.