"The thing is I am enjoying myself. If we won the Champions League, I would probably enjoy it even more.
"My time to go will be dictated to by my energy levels and my health.
"It is a fact of life that once you get into your 60s you are no longer in control of these things."
He added: "When I eventually go, I want to leave a really good team.
"Manchester United is a great club. It deserves my concentration to make sure whoever does take over inherits a good team. That is important to me."