From Longman Dictionary of Contemporary Englishfun runˈfun ˌrun noun [countable] DSOan event in which people run a long distance in order to collect money for charity
Examples from the Corpus
fun run• A two-and-a-half fun run will take place at 10.30am.• A fund-raising fun run is being held on Sunday.• Registration is $ 18 for the 10k race, $ 10 for the fun run.• Any child below the age of ten taking part in the fun run must be accompanied by an adult.• Monday, June 7 is the official entry closing date ... including the fun run. 