Normally Rose would have been grouchy when the "three hours" the Doctor had promised had turned into five. On top of that, her stomach was aching with hunger, her limbs were heavy with fatigue, and the steadily rising temperature was going against the whole Christmas theme she'd had going. But none of that could sour her mood when it was was buoyed up impossibly high by the alien walking beside her. She couldn't help a silly grin permanently gracing her features as she trudged along with the Doctors hand in hers. Not that he seemed to be able to help himself either. She caught him with a ridiculous grin more than once when she stole sideways glances in his direction. It was the same game they had always played, furtive looks and flirty chatter, but now instead of this dance leaving her frustrated and pining, it tended to end in snogging. Now that she thought about it, that could be why the three hours had turned into five. That and the Doctor's rubbish sense of direction.

"Well it's not my fault!" the Doctor scoffed at her when she brought this up to him. Then, his voice lowered to smoldering and his eyes grew dark as they held her gaze, "It's not my fault," he repeated, "that I have a companion who is so very…distracting."

Rose felt her insides turn to mush and then… well then they were snogging again.

"Mmmmm," Rose hummed when they came up for air, "We keep this up and we'll never get home." She didn't miss the way the Doctor's eyes danced when she referred to the TARDIS as such. "And that really, really should be a priority. What with the brutish locals and the needing to eat and the…" the rest of that thought fizzled out when her focus narrowed to the Doctor's kiss-bruised lips. She leaned in, but he just chuckled, pecked her on the lips and grabbed her hand to resume their trek. It took her a dazed moment to follow so she was a step behind him when he turned around and said, "Not the only one distracted now, am I?"

"Oh shut it!" Rose laughed and then sped up until they were side by side once again. She nudged his shoulder with her own and they both laughed.

"What you did manage to say, before you became overly fascinated with my mouth, is true. We really should make better time. Don't want the goons to find us and the constant rumbling in that stomach of yours is not the best soundtrack for a walk through a desert wasteland. Not to mention I can't just use regeneration energy every time we catch a chill. No, we best get back. Plenty of time for…other things once we're safe." The Doctor had looked at her in a way that would have made her tingly if she hadn't been focused on him bringing up how he had saved them the night before.

She walked a few paces in silence before asking, "How did all that work exactly? The regeneration energy thing, I mean."

The Doctor stared thoughtful at the horizon, "To be honest, I don't fully know. I thought I had just as good a change of triggering an immediate full regeneration as generating warmth for the ten or so hours we needed it. But I had to try something…" he trailed off before adding brightly, "But it all worked out, eh? Warm and toasty Rose, same 'ol Doctor."

Rose nodded absently. There was still something bothering her. "But there had to be some negative effects for you, yeah? I mean, as you said, you can't just go all glowy all the time. And in school…I learned something…something like energy can't be made from nothing? Is that right?"

"First Law of Thermodynamics!" the Doctor beamed at her and Rose couldn't help but smile even as she worried about what keeping her warm had cost her Doctor. "Energy can be neither created or destroyed. Einstein hardly needed any help in coming up with that one, the Third Law, however, well-"

"Doctor," Rose cut in, raising her eyebrows meaningfully. She wasn't going to let him off the hook so easily.

"Right, sorry. You, my brilliant Rose Tyler, are correct all around. The energy needed to come from somewhere since there was nothing to trigger a natural regeneration. I burned up a few decades of my life to get things cooking and then went into a trance to contain it all. Easy! And since you are such a scholar of science now: 24.3 degrees of heat spread out over 9.52 Earth hours is the exact amount of energy of 33 years, 6 days, 23 hours, 21 minutes and 16 seconds of a Time Lord's life. Totally worth it!" This time Rose couldn't answer his wide grin.

"Oh," was all she could manage as her throat tightened up. 33 years!? A quarter of a human life just gone, and all the Doctor could do was grin and say 'Easy!' Did her really have so little regard for his own life?

"Rose," he said softly, stopping and giving her arm a tug to turn her around to face him, "I'm over 900 years old. I could live for centuries more, millennia even. 33 years is nothing, nothing to a Time Lord, anyway. Not to mention it's nothing compared to what I would give up to keep you safe, to keep you happy."

It was all Rose could do to keep the tears in. She stared down at her feet, unable to meet his eyes, "I would never want you to give up anything for me," she whispered.

"And I'd never want you to put yourself in danger, or leave a normal life behind, or see some of the worst things in the universe, but…well, it's little late for that. When I told you I love you…when I continue to tell you I love you, it isn't just saying the words. It's accepting all that comes with it. Why else did you think it took me so long?"

"Dunno, just thought you were repressed or something," Rose mumbled, but she felt her lips turning up the tiniest bit. The Doctor chuckled, then took her other hand so both of hers were gripped in his large palms. He swung their arms back and forth.

"Rose, look at me so I can see that lovely simile you're fighting," the Doctor said tenderly, but with restrained humor in his voice.

She finally met his eyes and caught her breath at what she saw there.

"I love you. And for some barmy reason, you love me too. So this? This is what we do. For each other. Because we can't bear not to. Alright?" he asked.

"So the next time I come after you to break you out of some alien prison, you won't be cross?" she asked, biting her tongue as her smile widened.

"We'll see," he chuckled and dropped her hand to wrap her into an embrace.

"Hypocrite," Rose laughed into the soft leather of his jacket.

"One step at a time, yeah?" he tried to justify.

Rose pulled back to look him in the eyes, "One step at a time," she agreed. The guilt was still there, nestled in with the fear of all that he would do for her, but it was pushed into the background. Because he was right. She would do anything for him too.

And then, they were snogging again.

So much for making better time.