Jack's had a fair bit of practice over the years at managing to stagger distances much greater than this in far worse physical condition. And he has Ianto to lean on, which makes it a lot easier. He collapses into the helpfully opened up bed, and then whines when Ianto doesn't follow him. Even if it's for as altruistic a reason as taking off his clothes so they can have skin to skin contact, he misses the contact now and it's an effort to be patient. The fact that Ianto's being so hasty that he's just flinging his clothes down does help a bit, but he's still shivering again before Ianto's finished. He clamps his teeth together to stop them chattering and refuses to whinge when he knows Ianto can't physically go any faster.