I'm pleased to report, thanks to a 9 minute mile pacer from Runners World, that I achieved my target time of under 4 hours, (3:56:55).
The pacer, whose name I unfortunately didn't get, guided us through a treacherous course of discarded black sacks at the start, around, between, and sometimes over bollards, and weaved his way through innumerable runners to get us to the finish on time. All us '9 minute' milers had to do was squeeze our way through to get there with him.
The biggest motivators were:
1. A banner at about mile 24 reading "Impossible is Nothing" which sent a tingle down my spine and urged me on to the end.
2. The crowd in the tunnel at Canary Wharf
3. Friends Emma and Alex with their "Come on Steve" sign which appeared at two markers
Thanks for everything