Sir Alex Ferguson is thinking back to his first match in charge – "we bloody lost" – and trying to explain why it is, on the eve of his 25th anniversary at Manchester United, that he still has such a remarkable appetite to continue in the job. "I think another three or four years yet," he says. "It is still a long way off, believe me."

He talks of the drive that enables him, not far off turning 70, to get by with such an unfeasible lack of sleep: in bed by midnight, into Carrington before sunrise. Then he breaks off from the questions and directs his audience to the photographs on the walls before offering the most illuminating insight into why he does not want to let go.

This is United's youth-academy building, and it is here you can see the pictures of the golden generation – David Beckham, the Neville brothers Phil and Gary, Paul Scholes and Nicky Butt – holding aloft some of the 37 trophies that have been accumulated on Ferguson's watch.

"Was that a one-off?" Ferguson muses. "No, it's not. It is going to happen again. You can't think that Manchester United could have only one cycle of players as good as that. We will always keep chasing the dream."

The truth is that the future excites him too much to give it up. United won last season's FA Youth Cup, with exciting talents such as Paul Pogba and Ravel Morrison emerging. Ferguson wants to be there to see if they can make it. "We will get a bunch like that again. We have to."

He talks with such great confidence that it feels almost impudent to point out that Manchester City are threatening to deprive him of a happy ending. Of Ferguson's quarter of a century at Old Trafford, there have been few more harrowing defeats than the 6-1 against City two weekends ago. He addresses it matter-of-factly: "The challenge is always here, in this place, no matter who you are up against."

These days, he does not even want to think about a retirement date, and anyone who pushes him on the subject does so in jeopardy. The temper is still there, even if Ferguson describes himself as a "pussy cat" these days. Witnesses report he "turned purple" when the comedian at his anniversary dinner told a joke about Ryan Giggs.

The next morning back at Carrington was spent addressing Morrison's decision to write "piss-take" on his Twitter account after not getting into the reserve team on Thursday evening. "There is always something," Ferguson says. "Every day brings something new."

But he "likes being around young players, prefers young players" and, later, using the example of Sir Bobby Robson, he opens up about some of the qualities that he has needed to be at the higher end of the business for so long.

"He [Robson] had all his health problems, including two or three different bouts of cancer, but it never stopped him. Most people would have been just happy to get over that and have a nice easy life. But Bobby, right to the very end, wanted to come back into management. That sort of enthusiasm is incredible. It's a gift. People don't understand it is not easy to work hard at 70-odd years of age."

Ferguson was 44 for that first match at Oxford United. "Of course I can remember that day," he grizzles. "Two-nil. I said to myself: 'Oh, Christ almighty, I've picked a job here, all right.'"

But the changes between the Manchester United of then and now are significant. "It is a massive club now," he says. "The greatest thing we ever did was move [the training ground] from The Cliff to Carrington. We have 110 staff at Carrington now, excluding the players. One hundred and 10!"

The most important of them all takes a couple more questions and then rises to his feet. "Hasta la vista," he calls over his shoulder.