"I know. But you'll find time doesn't move quite as fast as you think it does," he murmured, pulling back just enough to look in her eyes. "Your folks will be here a long while yet. And that's a bridge to cross when you get to it, not look down the road for."
The Doctor tried to not detach from emotion when he explained the plan. "It's sort of an addition. An augmentation; human plus time lord. Couldn't just replace it all, this is more of an improvement. We evolved this way because of our proximity to the Time Vortex for billions of years. You've spent enough time to already have been changed. Should be easy enough, just helping along the process. You're already a time traveller. Just will change a bit more."