To complete a marathon means to run or walk a very long race, called a marathon, from start to finish without stopping or giving up. It usually covers a distance of 26.2 miles (42.195 kilometers). Full definition
Wear that medal, because not many people complete a marathon in their life, but you did! (richardblais.net)
Congratulations on completing the marathon, Even though I also love running (because I can clear my mind and get fit) I don't think I'd like running in a marathon. (jenloveskev.com)
They were tested again within 48 hours after completing the marathon, including a second MRI study and final blood sampling. (sciencedaily.com)