Jeremy Corbyn has vowed to serve 10 years if he becomes PM.

His Labour Party will be in peak fighting form by Christmas with candidates in nearly every target seat.

But if Theresa May clings on until 2022, when the next election is due, he would be 83 before he left No10.

In an exclusive interview with the Sunday Mirror, the Labour leader said: “I’m very fit and I’ve grown into the job. I enjoy every day and I know my own mind.

"I’ve waited all my life to see our country transformed, to bring about social justice, and I’m relishing the prospect of government.”

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Mr Corbyn certainly seems to be enjoying himself. He may be 68 but he bounces into the room like a man half his age, full of gags and good humour.

It’s a far cry from the Mr Corbyn I met during the leadership contest two years ago. There were no gags then.

He admits it’s been a steep learning curve. saying: “I had to get used to the speed, the timetable, the pressure and to understand government. I’m now better prepared to be PM than ever.”

He will need time to solve long-term problems like the housing crisis, transport and business investment.

But speaking as the Labour conference opened in Brighton, he said there was much he could do quickly.

A priority as PM would be to lift the one per cent public sector pay cap, meaning if he was in power nurses, police, firefighters and local authority workers would get an immediate 2.7 per cent extra in line with inflation.

Their future pay would be determined by collective bargaining between management and unions.

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He would also press ahead with plans to curb excessive pay by making bosses stump up an extra 2.5 per cent tax for any employee earning more than £330,000 and five per cent on pay packets of £500,000 or more.

The minimum wage will be even higher than the £10 an hour he promised by 2020 at the election because of inflation. And he has not ruled out a maximum wage. He said: “We are looking at more details of the policy.”

That would mean no boss could earn more than 20 times their lowest paid staff, capping salaries at around £500,000. Mr Corbyn said: “Just look at the £11million chief executive of McDonald’s. For every worker who gets £1 every ten minutes work he gets £1,300. It’s a big difference.”

Amazon, Google and Facebook are also in his sights for not paying tax here and dodging the minimum wage. He said: “Big business make huge profits which go into a tax haven so we can’t get tax income from those profits.

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“Tax avoidance means underfunding of health and education. I read about Amazon charging its workers in the West Midlands £8 to get to work, meaning they take home less than the minimum wage. It makes me furious. It’s a disgrace. It shouldn’t be happening and under a Labour government it won’t.”

There will be an immediate public inquiry into tax avoidance and, as PM, Mr Corbyn will ensure the minimum wage is enforced.

The pay gap between those at the top and those at the bottom has doubled over the last 25 years and people should be angry about it.

Mr Corbyn says they are: “It’s a silent anger that they’re being short-changed by the very wealthy in our society.”

Perhaps the wealthy should feel guilty, I say.

“I don’t know many wealthy people,” Mr Corbyn replied. “But some wealthy people do tell me they’re embarrassed by their wealth.

“They are well able to pay more and they don’t have a problem with us increasing taxes. Some are happy to pay it. Some of course aren’t.

“Even the very wealthy who have a heart attack need an ambulance and if their mansion catches fire they are going to need a fire tender.”

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Mr Corbyn goes into this year’s conference in a very different position to the one he was in last September.

Former shadow Cabinet minister Owen Smith had just failed to oust him in a humiliating leadership challenge but moderate Labour MPs, and that was most of them, still thought Mr Corbyn was a loser. They were convinced he would be trounced by the Tories at the General Election and probably destroy the Labour Party as an electoral force in the process.

His astonishing success, and Theresa May’s equally surprising failure, meant he saved many of their jobs and they have now gone quiet.

As The Economist magazine said: “His room for manoeuvre expands by the week. June’s experiment with diluted Corbynism was a success. Expect the next dose to be stronger.”

That does not mean Labour MPs love Mr Corbyn, but they accept he is now their best hope of power. And they recognise his extraordinary popularity, especially among the young who sing “Ohhh Jeremy Corbyn ” wherever he shows his face. No one ever sang “Ohhh Ed Miliband.”

Last year 18 parents named their baby boy Corbyn. The word Corbyn fans most use about him is “principled” and that is his great strength.

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He says he is also beginning to capture the middle England Tony Blair spent so much political capital on wooing. Mr Corbyn added: “More people registered to vote in the last election and we put forward a very different manifesto to the Tories.

“The best support was from young people but we also won constituencies it seemed impossible to win. A lot of the middle classes were voting Labour.” All that has emboldened him to strengthen the grip of the left with proposals for rule changes reducing the number of MPs needed to endorse leadership candidates.

Mr Corbyn’s life will also be made easier with more of his allies on Labour’s ruling national executive.

Although the Labour leader needs to capture 64 seats for an overall majority, he only needs to replace seven Tory MPs to become PM in a coalition, probably with the SNP .

In his sights are Tory big beasts, the Home Secretary Amber Rudd , Education Secretary Justine Greening and former leader Iain Duncan Smith.

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Scotland remains Mr Corbyn’s best hope of grabbing power. Although Labour won 41 of 59 seats there in 2010, it was virtually wiped out north of the border in 2015 and now has just seven MPs, behind both the SNP and the Tories.

But there are 20 Scottish seats where he is just 4,000 votes away from victory. Mr Corbyn is looking forward to an election and the sooner the better.

That could be any moment if Brexit brings May down. But her much-heralded speech in Florence has not changed Labour Brexit policy.

Mr Corbyn agrees with Mrs May on a two-year transition period, or rather that she now agrees with him.

I suggest that Mrs May is like a frail, elderly relative about to die but no one knows when. He frowns, saying: “She faces enormous difficulties in her party. But I don’t do personal attacks. I can’t best George Osborne on that.”

But in electoral terms, the PM could well soon be cut up into little pieces in Mr Corbyn’s freezer.