need 4 speed
I've done 4 x 1 mile a couple of times. That is a tough workout and not one I would like to do again to be honest. I'm a lot better at 800s and 4 minute repeats (so just a bit longer than a km). I asked about the 400s because I can maintain a fast pace easily on those... next easiest is 2 minute repeats and 800s.