The British Heart Foundation used to do an annual off-road across the Kent Downs, they have three distances, 25, 50 and 100 miles if I remember correctly (we've been discussing this as next years event, either that or Devizes/Westminster Canoe Race).
I'll have a dig around for you, meanwhile you could do worse than look at their website, they've got a few lined up for the rest of the year
http://www.bhf.org.uk/get-involved/even ... rides.aspx