If it's in the same day, it's a century. Walking 24 miles isn't a marathon because a marathon is a running race. A century is a 100 mile bike ride, not a race, who cares if you cut it up in segments? Actually, who cares what you or I or anyone else calls it? I call it a century to give myself a mileage goal for the day, I'm not putting it up on some Galactic scoreboard.