Run, Fat Boy, Run (2007)

Dennis (Simon Pegg) is a clueless, bumbling overweight security guard. He works a dead end job, can't ever pay his rent, and can't maintain a relationship. Five years ago he left his beautiful pregnant fiancée (Thandie Newton) at the alter. It isn't until she moves on and starts dating a successful, American businessman (Hank Azaria) that Dennis realizes he made a mistake. Dennis can't seem to compete with the accomplished new boyfriend. He tries to win her back by attempting to run a major marathon with only three weeks to prepare. He is aided by his gambling addict friend (Dylan Moran) and his hefty lovable landlord (Harish Patel). Dennis tries valiantly to win back the love and respect of the woman he loves by finally finishing something that he set out to do.