Didn't complete :(
I found another Walk-the-Walker at the beginning and we kept similar pace for a long time together. I was really pleased, I was steadily knocking off miles at 14.5 min/mile, and the route doubles back on itself, so we had a lovely long walk from ~2 miles to ~5 miles with faster people coming the other way. The atmosphere was amazing, and the scenery fantastic - we went under the Clifton suspension bridge and along the gorge, then turned and came back again.
7th mile was 15 min, and I never saw the marker for mile 8 but at mile 9 the lap time was 32 min. Somewhere between 7 and 9 my partner drew ahead and I just couldn't keep up with her. By 9 miles I was limping and I limped to the 10 mile marker in 21 minutes and soon after that I started feeling quite woozy, but I remembered a St John's Ambulance point ahead, so I struggled that far and then stopped.
I explained I wasn't injured, just out of steam, and they gave me a space blanket and said they'd call me a minibus to get me back to the finish. With one thing and another, that took over an hour, during which I occasionally stretched and moved around, and generally tried to recover. Tony sweet-talked the hotel into extra time on our late checkout, so I had some time to shower and change and shove some food in before getting back to the station and our train home. That had to be on foot, as most of the roads were still closed for the race, and I'd seen from my lift that traffic was awful. So one extra mile at quite high speed and now I am comfortably sat in our reserved seats on the train eating enormous quantities of food.
I'm quite disappointed that I didn't complete, but I'm not put off trying something similar again in future. I clearly haven't got quite enough stamina, but hopefully training regularly will build that up. I need better walking shoes as blisters have been a problem on every walk over 6 miles so far, so a trip to get proper gait analysis etc is on the plans for the next credit-card period.
Onwards and, well, onwards. I'll probably go out training again Tuesday or Wednesday.