"She had a sort of immortality already, in a roundabout way. Even if she died, here and now - which, obviously, she wasn't planning on doing for a moment - in just under 2,000 years she'd be back on Earth, wandering about London, growing up. Almost 200,000 years after that, she'd be on a space station, defeating the Daleks. More years than she could comprehent after that, she'd be watching the Earth die."