Ed Miliband has confronted the growing concerns about his leadership qualities, defending himself against the charge that he has allowed a policy vacuum to develop within Labour.

Ahead of David Cameron setting out plans to curb executive pay this weekend, he has also ridiculed the idea that the prime minister can match his determination to forge a new kind of responsible capitalism in Britain.

In a Guardian interview, Miliband said: "If one of the big battlegrounds of British politics is going to be who is really going to take action on executive pay, I say 'bring it on'. I promise you they are not going to steal a march on us in this area.

"Does anyone really believe that David Cameron came into politics to create a more responsible capitalism? The public are not going to buy it."

He defended himself against the charge that he lacks the decisive qualities to lead the party from opposition into government in one term. "These are the hard yards of opposition. We have taken the hard road, not the line of least resistance. I think it is a fight. I always knew it was going to be a fight. It is one I relish – I never expected it to be anything else."

The comments follow claims by an exasperated former policy adviser, Lord Glasman, in the New Statesman this week that Miliband has appeared to exhibit "no strategy, no narrative and little energy". Glasman said: "We have not won, and show no signs of winning, the economic argument."

"Look, I am the guy who took on Murdoch," Miliband said on Friday. "That was a decisive thing to do. I am the guy that has said the rules of capitalism as played in the last 30 years have got to change. What is the most important thing for a leader of the opposition to have? It is to establish an argument about what is wrong with the country and what needs to change. I have a very clear plan and I have set out very clear themes."

Miliband is due to make announcements next week on how the party will adjust to the reality that a fair society can no longer be created simply by redistributing the proceeds of growth, the model that worked for previous Labour governments. He said: "That used to be the terms of trade. That is what Blair and Brown did between 1997 and 2010 – new schools, new hospitals, tax credits – that is not going to be available to the next Labour government. The Blair-Brown route is not available to us. Social democrats are facing problems with this all round the world: difficult times don't automatically mean social democracy wins."

He also rejected suggestions that he was in denial about the deficit, saying the shadow chancellor, Ed Balls, "is the guy that invented the spending freeze 1997-98".

He praised the shadow defence secretary, Jim Murphy, for setting out how the party could make £5bn in defence cuts, saying other shadow cabinet members had made similar pledges and he wanted more to follow Murphy's lead.

Murphy warned this week that populism on spending cuts would not lead to popularity. Praising the detail in Murphy's plans, Miliband said: "It is exactly what Ed Balls said he wanted shadow ministers to do – to show that we do not oppose all the cuts. There are lots of cuts we are not going to be able to reverse. That is the way it is. To say otherwise would not be credible."

But he rejected suggestions he should set out new fiscal rules now in an attempt to bolster the party's trust ratings. "This process of restoring Labour's economic reputation and winning the economic argument takes time. It is an incredibly important task, not something that takes place in six months or a year. We – Ed and I – get this more than anyone, that this is an important task for Labour." Miliband also insisted he would challenge Cameron and Vince Cable repeatedly on the terrain of "responsible capitalism". He said: "On energy bills, rising train fares and banks, this government talks a good game sometimes but only ever fiddles at the margins.

"My test for him on executive pay is whether he would do what Labour would do if we were in office now: put an employee representative on every remuneration committee, make firms publish their pay ratios, empower pension companies and investors, and have another year of the bank bonus tax to get some of our young people back to work.

"There can't be any more foot-dragging and backsliding. If David Cameron wants a more responsible capitalism – responsibility at the top and the bottom – then this would be a start."