Seven Dials (where seven streets meet in Covent Garden) with the Christmas lights and lots of people doing interesting things.
Walking between the matinee of Apologia at the Trafalgar Studios on Whitehall, Stockard Channing starring, and Skerryvore at the Slaughtered Lamb in Clerkenwell (although I ended up in the wrong pub first and had another brisk walk - which meant I added lots of steps to my woeful total).