Steven Moffat has hinted at what happened with John Hurt’s War Doctor following the Eighth Doctor’s (Paul McGann) regeneration at the end of “The Night of the Doctor.”
Speaking to Doctor Who Magazine, the showrunner said:
“I assume what’s been happening during the John Hurt years that we never saw, is that he battled hard and fiercely in a way that the other Doctors would not have done, and that he was a dangerous and difficult man. But in his view he was not living up to the standards. I mean, by involving himself in an ongoing war – I always thought that sounded odd.
“I remember when David Tennant said, ‘I fought in the Time War’. I thought, ‘The Doctor in a war?’ I mean, the Doctor may be saving people at the fringes of a war, or stopping a war, but I could never imagine him being in one. But John Hurt’s Doctor is the one who was.”
We know from the first vague look we had of John Hurt’s War Doctor, right after Eight regenerated in the fantastic minisode "The Night of the Doctor"—which finally heralded the return of Paul McGann as the Doctor (one of the best things about the 50th-anniversary celebrations, in our humble opinion)—that he was much younger-looking than what we saw of him in the 50th-anniversary special “The Day of the Doctor.”
Obviously meaning that he indeed fought very long and hard.
But do you agree with Steven Moffat's view that no other Doctor—except John Hurt's War Doctor—could have been and fought in the Time War?
(via Doctor Who TV)