Sir Keir Starmer said he would face a “difficult inheritance” and would need to do “tough things” if he becomes prime minister later this week.
He acknowledged that after “14 years of failure”, he would not be able to “get a chequebook out on the Friday after the election” to boost funding for local services.
The Labour leader said there were no “no-go areas” for his party as he campaigned in traditional Tory seats in an effort to secure a landslide victory.
Sir Keir, who has repeatedly stressed how he had changed the Labour Party since taking over from Jeremy Corbyn, said he would continue to insist on discipline from his MPs if he became prime minister.
He was speaking during a campaign visit to the Buckingham and Bletchley seat, which would require a 12.7-point swing to win and has symbolic importance in this election.
It would be the 125th Labour gain by size of swing, the one that would represent the point at which Labour had an overall majority in the Commons.
Sir Keir said: “There are no ‘no-go’ areas. And I want to come to places like this, places that we wouldn’t normally win.”
He said there were Conservative voters who had “become disillusioned with the Tory Party now, after 14 years of chaos and division”.
He added: “I haven’t in any constituency, this one included, yet encountered anybody whose response to me has been ‘things are pretty good, I really like it, I just want more of this’.”
But he acknowledged the scale of the challenge he would face if he finds himself in No 10 on Friday.
He said: “If we do get over the line and come in to serve as a Labour government, it’s going to be really difficult because there’s going to be a very difficult inheritance after 14 years of failure under this government, and we’re going to have to do really tough things in order to move the country forward.
“I’ve never pretended otherwise, there’s no magic wand.”
He added: “After the first five years of a Labour government I want to ensure that all of our public services and all of our councils have the funding they will need.”
But he said: “I’m not going to be able to simply get a chequebook out on the Friday after the election and find money that isn’t there.”
Sir Keir defended the “steely” approach he had taken to reform the Labour Party following defeat under Mr Corbyn.
He said there was a “broad church” of views within Labour, but it could “never lose sight of the objective” of being in power.
He added: “I want a decade of national renewal and I’m not not going to be deterred from that, I’m not going to lose focus or discipline on that.”
Mr Corbyn told the i newspaper that Sir Keir had imposed a “straitjacket of conformity” on Labour and he would “crush dissent” in office.