Competitive swimmers can easily swim 2.5 miles without even training for it. Even if they haven't swum in years. Their times may not be the best, but they have a huge advantage over beginner swimmers. Competitive swammers know how to move through the water without much effort. The slower they go, the easier it gets. That doesn't happen to people with little experience. 

But there is a way that in just 5 months a beginner can beat a competitive-out-of-shape swimmer in a 4km race.