I like session 1, it's simple and short. The aim is to work as hard as possible every alternate length. Although Julian says to ignore the time, I do like to note whether I've beaten the 2:00min/100m barrier that seems to exist for me. Well for 17 out of 18 lengths I did achieve it.
Then I went to finish off with 400m alternating fast and slow without a break between them. I only surpassed 2:00min/100m for 1 of 6 lengths this time but the set came in at 9:04min for 400m which at one time was the best I could manage for my 400m time trial at the start of a session. There is progress.