What is the magical number 5K all about? Around about 3 miles. Is it just because it's sort of a round number being half of 10K, or that it chould be about half an hour or so or what? If we weren't metric would we be doing the couch to 3 miles (c23m) or would we be trying to do 5 miles in an hour? Is it healthcare bods saying we should exercise for 30 mins 5 times a week (or however many times it is)?