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. 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.
