I wasn't feeling the best before this workout, but I have to say, I was excited to hear what we were doing. Anyone who calls themself a 10K runner should love 20 by 400 w/ 30 seconds rest. We alternated directions every 5 laps which was nice. I felt like it did a good job preparing me for a real 10K. If I learned anything, it is to just get into a rhythm running 69s or 70s depending on how I feel, and not go much faster (even a 65 lap could take it out of me and make things worse for several laps). I ran 69, 68, 68, 68, 68, 69, 69, 70, 68, 68, 68, 67, 67, 67, 67. That was all the laps my watch could hold but afterward I probably ran 67-68 per lap with a 66 and 64 for the last two I think.