The South Downs Way I have learnt is 100 miles, and two thirds as long as its nothern equivalent, though has been used for a little bit longer (some 8000 years). A really good walk for a day or two.
Between Seaford and Eastbound there are the seven sisters which are chalk cliffs. There is supposed to be seven but there is an eighth on its way due to erosion of the sea. Do not let the eye fool you though, these hills are a lot steeper than they seem. Plus just because you have finished the sisters does not mean you are done the ups and downs!
At the end, in Beachy Head there is a reward as you can have a cold pint in the pub. Though thus place is also known for being a popular spot for suicides and also being featured in many films like Quadrophenia and most popular, the Quidditch World Cup in Harry Potter and the Goblet of Fire.