@fancyoats, can I ask how "intense" you perceived this exercice ? More specifically, do you feel that you could for twice as long, or 16K, at the same pace ?
The reason why I'm asking.
I am week behind you in the BPP. Sunday and today I did respectively W6S1 and W6S3 (7.5K), with W6S2's intervals yesterday.
My data for W6S1 and W6S3 are almost identical in terms of pace and H/R:
Iain, if you read this I think you will understand where I'm coming from right ? I have slowed down these steady sessions where I think I could extend them longer. You had recommended to do them at a pace where I could double them, but I'm concerned that if I do that, I would need to do them at such a low H/R that I wouldn't get much benefit. So, so far I have reduced the pace where I could extend the row "by some amount" which I can't quantify exactly, but I still get my heart pumping. See also the below for what I mean (this is after 10' warm up).
Perhaps you would like to comment further on that.

48yo French living in Hong-Kong / 168cm height / 55kg / BMI 19.5 / Concept 2 PM4 / Garmin FR255 / HRM-Dual / MHR 182 (seen) / RHR 55