"Hardest" is open to your interpretation. Can be anything from 100 x 100s to 8 x 50's all out on 1:30 test set.
Repeat 400 IMs!
wow
I recall Jim Thornton relating a tale of a masters team-mate doing 11x400 IM in an hour. That a 5:27 interval. The most I have ever done in a single workout is 4, but never as repeats.
