Dennis Doyle is about to marry Libby, his pregnant fiancée. However, he gets cold feet and runs away during the preparation for the wedding. Five years later, his life has gone in circles ever since. When Dennis discovers that Libby has started seeing high-flying go-getter Whit, with their son, Jake, he realizes it's now or never. He finds out that Whit is running the Nike River Marathon in London, and to prove himself to his doubting friends and, most importantly, Libby and his son, he decides to run the race himself. Dennis receives motivation from his two "coaches", Gordon, his best friend who has made a hefty bet on Dennis succeeding, and Mr. Ghoshdastidar his landlord, who uses unorthodox methods of training, such as a spatula.