Let's start off with the South Downs; they are the lumpy bits in the South of England near the coast below London. More accurately they stretch between Eastbourne and Winchester forming one of England's National Parks.
|The South Downs Way|
The South Downs Way is the longest bridleway completely within a National Park, covering 100 miles between Eastbourne and Winchester. It is virtually all off road going over the tops of the South Downs and into the valleys to cross the rivers. The amount of hill climbing is around 10,000 - 12,000 feet. (That's a lot for 100 miles)
The National Trust says it is possible to cycle the length of the South Downs Way in 2 or 3 days. They also say "It is possible to do it in a day but that’s an extreme challenge!"
The South Downs Double is the challenge of starting at one end, riding to the other, and then riding all the way back. Only 24 people have been able to complete this with just 16 riders achieving a sub 24 hour time. In 2011 there was less than a 30% success rate.