We had the best workout today!! It was 4x400 all-out with an 800 jog between each (which might not sound hard?? but it was). They weren’t exactly all-out; they were supposed to be somewhere between 5-10 seconds slower than your 400 PR which for me is a high 64, so I did these perfectly. I did mine in 70, 71, 69, 68. I went out too fast on the first one so I was inconsistent with my splits there, but the other other three I either split negative or even.  Yay for improvement