The Dark Knight Rises Question on Ending (Spoilers, beware!)

[Adamlee]

So I understand that Bruce Wayne fixed the autopilot, enabling The Bat to take the nuclear bomb out to sea. But I wonder exactly how he was able to survive a bomb blast, which Dr. Pavil says has a six-mile radius, when he has less than two minutes to get it out over the sea. How fast would The Bat had to have been going in order to get a reasonable distance from him, assuming he hit an eject button into the water? This is the one big issue I had with the film.

I assume he also fooled Gordon and Kile into believing he was going "without an autopilot" so that he could start a new life for himself.