I've now completed W6R1. It felt like a bit of a slog - my breathing was OK but my legs were very tired, so I went super-slowly. I took on board what so many other people have said that this is hard work after the 20-minute run, so I took it very gently.
I also tried a new route - up the canal to Three Mills (where they film Masterchef) and around the little park there - so I had some new scenery to distract me from the effort.