some of paula's friends did a similar thing on the Gower, was great fun because nobody had a clue and they split friends who new each other up...
also did the national welsh motorcycle rally way back, did 740 miles in 24 hrs on 3 hrs sleep, finished last check point with 25 mins to spare
thing is you announce the locations (ie town/villages) a week before the event, then people pick their own routes, you hand out the clues for information to collect from each location at the start point along with a list of optional general things to collect along the way.
Awards for finishing all checkpoints (not timed) and extra awards for collecting all the extras.