His comfort is more precious and soothing than she could express in words or thoughts, so she simply answers him with a sense of gratitude. It enables her to tell him the rest, which is still distressing her badly, second only to being cut off from her own Doctor.

"I can't draw energy from this universe at all. I am only alive because I am constantly siphoning energy off of the Rift." She leaves the implications unspoken, but they're plain - she has to ration her power, she can't leave the island, and she probably shouldn't attempt any significant hops in time either. Finding her hasn't made him any less stuck, and she abhors being useless like this. Not to mention if anything should happen to the Rift, she's at the mercy of the consequences.

At this point she leads him off the path and up a small hill into the woods. Behind the hill, the blue of the police box is easy to spot through the mostly bare vegetation.