In four stages, with other charitable types, I shall be walking all the way round the capital city - c.120km in total - while hopefully raising money for Mencap. https://www.mencap.org.uk/event/capital-challenge

Stage 1 (Hendon to Royal Albert Docks) is Sat 5 May.
Stage 2 (Royal Albert Docks to Crystal Palace) is Sat 26 May.
Stage 3 (Crystal Palace to Richmond) is Sat 9 June.
Stage 4 (Richmond to Hendon) is Sat 7 July. 

Each leg is around 30km (c 20 miles), easily farther than I've walked in one day before, but it's worth it.