Sprint interval mistake

I did some sprints the other day. Turns out I did them wrong, thanks to a friend for pointing this out.

I did 4 sets of 4 reps of :15on :15off with 1 min rest per set.

The sprints called for in the book have a work:rest ratio of 1:6-9. So it's no wonder the first one of each set was strong but every other one was terrible.

Next time I'm doing :15on 1:30 off.