The leap from 9 to 16 minutes running proved to be a bit too much; I'm afraid I sneaked about a minute of walking into the first 5 minute run - I was going slow, but probably not slow enough and I clearly haven't learned to pace myself properly early on. I did the rest (incredibly slowly), so maybe 15 minutes, and still an enormous improvement from when I started.
Well done to those who made the W3 - W4 transition first time. I'll have another go (furtively, in the dark, of course) in few days when my body has recovered.