Run Fat Boy Run is a 2007 comedy movie directed by David Schwimmer.
The movie starts with Dennis about to marry his pregnant fiancé Libby, but he ends up running away after getting cold feet on the day of the wedding. Five years later, Libby has given birth to their son Jake and is now dating Whit, an athletic go-getter about to run a marathon in London. Upon learning this, Jake decides to run in the same marathon, determined to prove himself to his former fiance and son, enlisting the help of his best friend, Gordon, and his comical landlord as coaches.
A few days before the race, Dennis is dismayed to learn Whit has proposed to Libby but he decides to run in the race anyway. Further to his anger, Dennis learns during the race that Whit plans on moving his soon-to-be wife and stepson to Chicago. As the competition between the two turns more and more aggressive, Whit purposely trips Dennis who is rushed to a hospital. Whit soon follows with an injury of his own, where he shows Libby his true colours by being aggressive and angry towards her and Jake. After he, Libby and Jake go home, they turn on the tv to find Dennis still running in the race, having refused to take medical help. They also see Whit tripping Dennis and angered by his attitude, as well as his intentions to take them to Chicago, Libby breaks off the engagement and rushes to meet Dennis at the finish line.
Dennis find himself struggling to finish the last leg of the race, but upon seeing Jake and Libby across the finish line he gets up in determination and crosses the line to victory. The movie ends with the three reuniting and Dennis and Libby rekindling their relationship.