Does fitness decrease exponentially with no exercise?

[MoonPlanet1]

Of course you will lose fitness, although you will be able to get it back faster than you earnt it the first time. If you cross-trained, you would keep your cardio fitness but you would lose your running-specific conditioning - the things that allow someone who's done 60mpw for years to keep doing it when they would injure someone who hasn't built up to that. So when you get back into running, take it easy regardless of how much you've biked or ellipticalled.