The Dunwich Dynamo, a.k.a. the Dun Run, is an overnight group ride which takes place every July from London[1] to Dunwich on the Suffolk coast. It is approximately 200km. Organization is courtesy of the London School of Cycling and Southwark Cyclists, and consists of route planning, ensuring there is a venue serving hot food somewhere near the middle, and optional coach transportation[2] for you and your bicycle back to London. There is no 'sag wagon'; you are responsible for your bike and yourself, though fellow participants may well offer a helping hand if you have mechanical difficulties. In 2013 there were over 2000 riders.
Photocopies of the route, which can vary slightly every year, are made available by the organiser. The terrain is not particularly arduous for cyclists of average fitness. It is not completely flat. Candles or hurricane lamps[3] are placed at various points along the way, and cheering or jeering groups of velo enthusiasts have been known to observe passage of the pelotons, particularly in Essex.
It is not a race, though competitive instincts are often in attendance.
The 2017 Dun Run takes place July 8/9, Saturday night and Sunday morning.
Notes
1 - Pub on the Park, Martello Street, London Fields.
2 - You pay for this. The ride itself is free. If you make the rounds of UK cycling forums you may find others organizing coach trips. There is a train station a few miles from Dunwich, traditionally staffed with conductors freshly shocked every year at the popularity of this corner of Suffolk of a Sunday morning. Booking ahead will provide some cover.
3 - Or something. Corrections welcome.