If you nailed your first marathon without even breaking a sweat, try this for size. The Thamespath Challenge is a two-day, 100km run along the Thames, from Putney to Henley. There's also a 50km option for those not quite ready to tackle the equivalent of two-and-a-bit marathons.
The city's longest organised run is about as tough as they come, taking you on a winding westerly tour through the suburbs and into greenest Oxfordshire. Despite the distance, walkers are welcome, with a 33 hour maximum for completing the course. For full details including start times see thamespathchallenge.com.