I don’t recall discussion around Ben doing 7 x 300, what I do recall is him hating 300 SE and Charlie dropping it.
I found this from Charlie re 300 tempo
Tempo 300’s:
I’ve used this session a lot over the years. Keep the recoveries at 100m walk (@=1-1:15min), but reduce the pace. 65% of max. is more appropriate, but be
honest when calculating your projected best time for the distance. It will seem
like you’re crawling pace-wise, but that’s OK; remember that by design this
session is meant to be aerobic. Since you moving up to the 400m, you may need
to start at 6 reps, and add 1 a week until you reach 10. If you are struggling with
6x300m at 65% of max, you need to try shorter distances (100-250m) first. The
transition to 400m is often difficult and not for every athlete.