Sure, it's not a great spectator sport unless you know someone who is running, but it is certainly a big draw. The marathon has been held in London every year since 1981.
If you are watching it for sport, as opposed to supporting a friend, your author understands the last five races have been won by Kenyans, so wont it be interesting to see if another Kenyan wins this year (probably not).
Tower Bridge and the Birdcage Walk/the Mall have always been amongst notable watching spots, but it looks the same from anywhere really!
For more information, visit the website at http://www.london-marathon.co.uk.