It's my favourite, you can do it in 2-3 hours and see almost all the best sightseeing of the city. Easily you can pass at the Borough Market for the lunch, sit out of one coffee shop in front of the Thames, watch a show of artists and pop in the Museum of Modern Art, the Tate Modern. London Eye, Big Ben, London Bridge, Tower Bridge and others.
For the amount of sightseeing to see. There is also the Shakespeare Globe, you can buy a ticket and watch a show.
Walk, just Walk.