A distance of 100 miles (161 km) is often referred to as a century. Century is also a common designation for 100 years, but that is not its exclusive meaning. The English word derives directly from Latin centuria, a group of 100.1 This is a form of centum (or CENTVM if you will) which simply means (one)hundred.
A ride of 100 miles is thus aptly named a century ride, without having to resort to metaphors or figurative meaning.
Chris H mentioned another instance where century is used to count something other than years: In the sport of cricket, a century is a score of 100 or more runs in a single innings by a batsman 2.