[ Admittedly, he's not always the most reassuring when he's worried. It's more of an urgent things are about to be very bad very quickly response that he doesn't always censor well in the moment. And while he can't actually be entirely sure of imminent death, any fog that burns like this can't be good to linger in, and a dire warning usually has the intended effect.

As the air around them thickens and clouds his vision alarmingly fast, the Doctor doesn't entirely make out the look of his new companion until they're only a few paces apart.

Running though, yes, good, this one listens! ]

If you lose sight of me, just keep running straight until you hit a solid door, that'll do it. Well, open it and get inside of course. Keep yourself safe.

[ Jokes, this one. Even now. Of course, he's also implying that should they manage to get separated in all of this, if the younger one at least just runs straight on towards the cabin, he should be okay if nothing else. The Doctor is obviously not intending for anything to happen to his own self in what should be less than a few minutes' to reach the cabin, but one never knows. ]

Just a lovely, mildly unsettling jaunt, won't be long!