Walking tours
The original London Walks company offers some great themed tours on a huge variety of subjects, including 'Jack the Ripper' and 'The Beatles'. Tours conveniently start and finish near a Tube station and on average last for approximately two hours. There is no need to book - walkers can simply just turn up at the appropriate station, all of which are listed on their website.
Offers 'Jack the Ripper' tours, as well as 'Haunted London'. Both take approximately two hours and depart from Aldgate station and St Paul's station respectively. Booking is required for the 'Haunted London' tour.
For a self-guided walk, the Silver Jubilee Walkway has been extended from the original three kilometres (two miles) along the South Bank of the River Thames (between Lambeth and Tower Bridges) to include much of the city and the West End. It now covers 23 kilometres (14 miles) and so, whilst it is still an achievable days walking, you may wish to consider breaking this up over two or three days.
Starting between Tower Hill tube station and the Tower of London, the three-mile London Wall Walk was created by the Museum of London and traces the original Roman wall of Londinium, some of which can still be seen today.