Somewhere along the way, he stops counting the years. He has pretty much lost track of how old he is anyway and years don't really matter in the same way when you're going to live forever. When he finally meets up with The Doctor he has no idea whether he's been travelling around for a few years or a few decades. It doesn't seem to matter.
The Doctor doesn't give him any real sympathy, not that he expected it really. He gets an 'I'm sorry,' but not much more. Not that there's really time for more, there never is. Before they can get much further than 'Hello,' the planet he's on is in danger again and there's no time for long conversations about things that can't be changed anyway.
He refuses to let The Doctor call him Jack. It was never his name anyway, just an identity he stole from a dead man. The problem is he can't remember his own name, his real one. Not now, it's been too long, too many years of answering to Captain Jack Harkness and too much time trying to pretend that's all he is and all he's ever been. They settle on 'Captain.' It's easiest and he tries his best to forget that there was ever anything that came after that title.
After enough years he stops turning round every time someone calls out 'Jack.' He takes his cue from The Doctor - introduces himself as just 'Captain' and tells people that that's all there is to it. Most people they meet find it strange - two men with titles but no names - but it doesn't matter, after all The Doctor is used to it and before long he finds he is too.
There are others who travel with them, humans and non-humans alike. They come and go. Some leave by choice, some are left behind whether they like it or not, some die. He doesn't let himself get attached to any of them. Most of them see him as rude and ignore him. Some take an interest in him and try to connect with him in some way.
It doesn't work.
He finds himself in bed with almost every single one of them at one stage or another, usually right after they save the entire universe. It always goes one of two ways; either they're scared and confused and too far from home looking for comfort and something steady to cling to, or they're on a high from the adrenaline and looking for a way to burn it off. Afterwards, he acts as if it never happened. Some take their cue from him even though he can see in their eyes that they want more. Others don't seem to care; they only wanted the one night. Some leave. He is genuinely sorry about those, especially when he sees the look in The Doctor's eyes and knows that the Time Lord knows exactly what chased them away.
The Doctor changes his face more than once. He never can. There's a little more grey creeping into his hair but otherwise he looks the same as he always did. Sometimes they'll visit places they've been before and people will recognise him, the look of puzzlement when he explains the man walking next to him really is The Doctor, the same one as before, is almost funny. Only almost. In reality he wishes he could change, could alter his appearance so dramatically that even he will not recognise himself in the mirror.
Eventually he starts to forget. He forgets their names - those people who died for him and because of him - but he can't ever seem to erase their images from his mind. There are so many, so many people who he watched die but a few haunt him more than others.
A woman standing in the night pressing a gun to her head.
A man bleeding to death from a gunshot wound.
Another young woman dying in his arms, her blood all over the floor.
A young man, cradled in his arms, looking up at him with such love as he takes his last breath.
A child, just a child, terrified and confused dying to save so many but never knowing it.
One of the Doctor's regenerations turns out to be a hypocrite. Not that this is entirely a surprise since most of his regenerations have been hypocrites to some degree since they started travelling together, but this one is a bigger hypocrite than the rest. After years of leaving people behind and running away, never looking back, this new Doctor drags him back to 21st century Cardiff without a single word of warning.
The Doctor doesn't take no for an answer, he never has. So Jack follows him out into this city he once knew so well, barely able to recognise anything now after so many years away.
He doesn't realise what The Doctor is planning until it's too late to do anything to stop it. It's not until they're standing outside a house he doesn't recognise, not until the door opens and a man is standing there, that he understands. The man's name lies half-forgotten somewhere in his mind, but it doesn't seem to matter because after the briefest of hesitations the man is shouting inside the house for someone to come quick. And then there's a woman.
It's been so long since he heard that name he barely realises she's talking to him. And then she's hugging him, her arms wrapped tightly around him. He hugs her back, acting purely on instinct and his brain, as though it has been waiting for the right moment, opens the floodgates and it all comes rushing back. Gwen. Gwen and Rhys. He remembers the others too, Suzie and Tosh and Owen and Ianto and Steven. All the others who died over so many years and those who survived. And Alice. Oh God, Alice.
Somehow he ends up sitting in their living room, Gwen beside him, staring up into his face and seemingly unwilling to ever let go of his hand again, Rhys hovering over the both of them. A child cries from another room and Rhys disappears. Gwen was pregnant, he remembers that, remembers her standing on the top of a mountain in the middle of the night begging him not to go. The Doctor is standing over him, looking down at him with an expression he can't even begin to fathom.
"A lifetime is long enough, Jack," the Time Lord says. "Besides, I do enough running away for the both of us."
Then the Doctor is gone. It's just him and Gwen sitting there in silence, the sound of Rhys soothing a crying child drifting to them from another room. After all this time it no longer hurts to look at her, he no longer feels the urge to get as far away as possible so that she will not be the next person he will lose. The sound of a child is no longer a horrifyingly painful reminder of the sacrifice he made.
"Are you staying?" Gwen's voice breaks the silence, a mixture of hope and fear in her voice. Her fingers tighten on his as she waits for his answer, as though if she just holds on she can keep him there. It's not needed though, not anymore.
"Yes, I'm staying."