Every year, the Tokyo Walk is organized in the various areas in Tokyo. I participated in summer 2010, when it was super hot and we almost got sunstroke (well not really but it felt like it). Walking among a group of sun-visored elderly ladies, we greatly underestimated the Tokyo Walk. But it put us in our place. There are routes of 7 km, 11 km or a pro route of 20 km, taking you through different areas. This way, you can participate up to five times a year and never walk the same route. In 2014, the dates are June 21st (Shinjuku & Roppongi), July 19th (Asakusa & Sumida river), September 20th (Hachioji & Machida), November 15th (Higashi Yamato & Tama), December 13th (Ueno & Kiba). Because the Tokyo Walk is not very well-known among foreigners, chances are that you are treated rather like a celebrity by the other participants.
Website: http://www.tokyo-walk.jp/ in Japanese
Tel no.: +81 (0) 3 5256 7855