Run Fatboy Run (2007)

Dennis Doyle, a lazy and out-of-shape man, realizes the error of his ways when his ex-girlfriend Libby, who is about to marry the successful Whit, comes back into his life. Determined to prove himself, Dennis decides to run a marathon to show Libby that he can change. Along the way, he faces various challenges, including a gambling debt, a pregnant bride, and his own lack of self-discipline. With the help of his best friend and a dedicated trainer, Dennis pushes himself to the limit to win back the love of his life.