Nothing is impossible.
I’ve never run a marathon before (or a half marathon, or a 10K, or a 5K, or... you get the picture) but I imagine there are points along the way where you think there’s no possible way you could ever finish. Your legs burn, you’ve got a cramp in your side, your feet are aching. Your situation seems just as dire as—let me pick something totally at random—a three-touchdown deficit in the second half of the Super Bowl.
And then you see that and you hear Kevin Garnett’s voice in your head screaming “ANYTHING IS POSSIBLE!!!” and you dig deep to finish the race.