I read someones blog about the sightseer from last year and it seemed that the time frame was pretty good, 62 odd miles in no less than 5 hours and no more than 10, though it was on a sunday which helped quite a bit I would imagine!
But that aside the route seemed pretty good and wasn't on many big busy roads, there's some river side riding and some park detours, one via Buckingham palace (que silly tourists and many cameras as a fleet of stunning recumbent machines roll past!) It seemed like a good way to spend a day.
I cycle around Cambridge everyday, through all kinds of traffic, and dont seem to have too much of a problem dealing with it! But Cambridge is not even slightly comparable to London, with the exception that they both have a river in the middle...
I think you have to pay a small extra fee to join the race for the day, bringing the total to about £5

the ride is going to be from near Hampton Court, across London to the Dome and back again.
here are some more details:
http://www.ctc-heartofengland.org.u...ightseer-audax&option=com_eventlist&Itemid=40
Hows the new trike Chonker? Treating you well I hope! How many miles have you racked up?