yep - there is a club/group/association/whatever that runs these things.
the biggie is the "sydney to melbourne marathon" which is about 900km
(about 600mile?), and is also entered by endurance riders, and i think
cyclists and stuff too. one of the guys who does it regularly has been
trying to get my dippa off me to train to harness and enter in it, but
he's not having much luck! :-)
nik & the gang (taaj, saki & dippa) in australia