If you can do just over 4.5 miles in the hour I'd go right round the Isle of Dogs on the Thames Path until you get to Westferry Circus and then walk through CW to get back to Churchill place. That's a very pleasant walk and can be shortened by cutting across the island as there's a footbridge over the docks at Pepper Street. This useful link might help you plan your route...
http://www.gmap-pedometer.com/

All the best - Ian