I've just got back from the 2nd run of W5. I found the 2 x 8 minutes a real slog - but I kept going doggedly to the end of each one.
Remembering what all you good folks have said about endurance, I did a few systems checks along the way and found that:
My legs were aching - but still good to carry on
My breathing was an effort - but not impossible
My pace was slow - but still "running".
I went along my beloved canal towpath again, and then along Narrow Street - which felt like doing the marathon, especially since the barriers are still there from Sunday. I finished with a run (i.e. jog/stumble) along the riverside, which was lovely.
Now just the 20-minute monster run to go!