6.5 miles is a long walk and not much point if it's only a whistle stop tour, I reckon you'll easily take over an hour to wander round the Tower, St Katherine Dock and Trinity Square, don't forget he's a country mouse so there will be a lot of Oohing and Ahhing at all they sights and sounds. Just a thought, as most of the participants are experienced at riding bikes and that the Capital is covered with safe cycle lanes, what about hiring Santander or even some of those wretched Lime bikes to tour on? You can take bikes on the river bus just need to polish up the drop off system. Or even better hire one of those cycling bar things, you shouldn't lose anyone and refreshments are within an arms length.