Taking this to a Physics formula of Time equals energy times efficiency over power equals (5670kJ)(0.25) over 800J/s equals 1770 s which gives the solution 30 minutes.
Therefore, it will take 30 minutes for a 68kg person to swim at a fast crawl in order to use all the energy available in a typical fast food meal of a burger, fries, and a drink. I estimated with a 25% efficiency for work capacity at swimming at a fast crawl in this equation.