Ed Balls says he has put ambitions to be Labour leader out of his mind because it was "very, very unlikely" ever to happen.
The shadow chancellor said he would still "love to be" Prime Minister but had learned to concentrate on reality.
And while he hoped to live next door as Chancellor after the 2015 election, he said he was content with what he had already achieved.
Mr Balls fought to succeed Gordon Brown as party leader in the 2010 contest won by Ed Miliband.
He was made shadow chancellor following the resignation of Alan Johnson, who has described tensions between the two men.
"In politics you never say never but I think it is very, very unlikely," Mr Balls said of his leadership ambitions on BBC Radio 2.
"I back him (Mr Miliband) 100%. I want to be the Chancellor of the Exchequer. I don't think there is going to be a vacancy and I think probably that is not going to be for me.
"In life, if you spend all your time thinking about what you might do or thinking about what you are not doing then you suddenly find out one day that you missed it.
"So actually one of the things I've learned, I've been in politics 20 years, is you should enjoy what you are doing and I don't think about it.
"Of course I would love to be Prime Minister. That's why I put myself forward for leader in 2010. It is the most difficult job in Britain and it is such an important job.
"But I am not going to get that chance and I want to be Chancellor."
He added: "If I end my career today, with all the things I've done, I'd be pleased with what I've done. But I want to be Chancellor."
Asked what those achievements were, he pointed to the decision not to join the European single currency, Bank of England independence, "transforming" NHS waiting times, the minimum wage and education to 18 for all.
"I've been involved in some important things. I think Britain is a better place than it was 20 years ago."
Mr Balls was on the Steve Wright Show as part of a bid to promote his latest London Marathon run this weekend.
He said he aimed to retain his place as the slowest MP ever to complete the course but the biggest fundraiser, having so far drawn in over £130,000 for the Action for Stammering Children and Whizz-Kidz charities in three years.