Run Fatboy Run (2007)

Dennis is a clueless and slightly overweight guy, who left his pregnant fiancée five years earlier. Every day, Dennis tries to persuade the woman he loves to accept him back into his life, but everyday he fails. When he discovers that Libby has found a partner in the form of American Whit, frustration grows, and Dennis vows, that for once in his life, he will finish something. This something ends up being a Nike River-run in London. With his friends Gordon and Mr. Ghoshdashtidar by his side, Dennis begins training for the marathon he must finish.