The BH5K Run is a well established and popular event.. Most competitors will run unclothed, but this is weather dependent; ladies may wish to wear a sports bra.
http://www.naturistfoundation.org/BH5K_naked_run/
NOTE CORRECTED DATE!!!
BH5K NAKED RUN
Would love to go – went to the BH5K last September – but I'll be on holiday.