A canoe marathon is a really long race that people do in canoes, which are like little boats you paddle with a special paddle. The race is usually on a river, lake, or other body of water that's really long and hard to paddle across.
During the race, the people in the canoe have to paddle really hard for a really long time. They might have to paddle for many hours or even a few days! It's a competition, which means they're trying to go faster than everyone else in the race.
Sometimes, there are different categories for the race. That means that some people might be racing in canoes that are made for one or two people, while other people might be racing in canoes that fit four or more people.
Canoe marathons are really tough and take a lot of work and practice to do well. The people who race in them have to be really strong, have good balance, and know how to paddle really efficiently. But it can be a really fun and exciting challenge to take on!