Speaking at the Tory Party Conference he said: "This is the moment when we all really have to try to make progress and get this thing over the line".
He earlier told the BBC he will offer the "good solution" to Brussels "soon", but stopped short of revealing exactly what his idea was, saying critics could “needlessly distort” what was being put forward.
His comments came after Dublin rejected reported proposals for customs posts along both sides of the Irish border to replace the backstop.
Irish state broadcaster RTE reported the suggestion sent to the EU by the UK would lead to the posts being built between five and 10 miles back from the current border.
But the prime minister told broadcasters the leaks were "confused" and claimed "they don't actually relate to what we are going to table".
He added: "One thing is clear, we don't want to see new border posts just away from the border - that's not what we're envisaging."
The reported plans by the British government to replace the backstop were dismissed by Irish deputy premier Simon Coveney, who said Northern Ireland and the Republic of Ireland “deserve better”.
The Conservatives have made a raft of policy announcements while at their conference in Manchester - including a rise in the national living wage - but they have been overshadowed by the PM's controversies.
He'd been dogged by allegations around his links to an American businesswoman while he was London mayor in the run up to conference but the situation was made worse when more accusations came on Sunday.
Journalist Charlotte Edwardes claimed he squeezed her thigh without her permission under the table at a Spectator event in 1999.
But the PM dismissed the allegations as "not true" and said it was "sad" that someone should make up such claims.
On Brexit Mr Johnson said: “They are not talking about the proposals we are going to be tabling, they are talking about stuff that went in previously.”
“But clearly this is the moment when the rubber hits the road," he told the BBC.
“This is when the hard yards really are in the course of the negotiations.
“The difficulty really is going to be around the customs union and to what extent Northern Ireland can be retained within EU bodies at all.
“We’re going to make a very good offer, we are going to be tabling it very soon, but there is a difficulty if you try to keep Northern Ireland in a customs union because one of the basic things about being a country is you have a single customs perimeter and a single customs union.”
Meanwhile, as Mr Johnson plots how he will take the UK out the EU, his opponents are teaming up to try avert a no-deal exit.
One idea that's been floated is a vote of no confidence to remove the government which would allow a temporary administration to take over.
But shadow chancellor John McDonnell has said opposition parties are now "unlikely" to table a motion of no confidence until after the EU summit on October 17 and 18.
Mr McDonnell was adamant that any interim administration following the ousting of Mr Johnson would have to be led by Jeremy Corbyn - despite the opposition of the Liberal Democrats and the rebel Tory MPs who had the party whip withdrawn.
On Brexit, the reported plans for the customs posts are contained in the so-called ‘non-papers’ submitted by UK officials during recent technical discussions.
Mr Coveney tweeted: “Non-Paper = Non-Starter. Time the EU had a serious proposal from the UK Govt if a #Brexit deal is to be achievable in October. NI and IRE deserves better!”
Shadow Brexit secretary Sir Keir Starmer tweeted: “If Boris Johnson had spent any time listening to businesses and communities in Northern Ireland, he would know that these proposals are utterly unworkable.”
The comments came after the Prime Minister said he remained “cautiously optimistic” about the prospect of a deal despite the limited time available ahead of the scheduled October 31 withdrawal date.
He said the UK side had made some “pretty big moves” towards a deal but it was up to Brussels to respond to find the right “landing zone”.
The moves came as it was was revealed that sexual and violent offenders will be kept behind bars for longer under plans being set out by the Justice Secretary.
Robert Buckland will confirm he will end the system which sees some of the most serious offenders released after serving just half their sentence.
The move – and a warning to criminals from Home Secretary Priti Patel that “we are coming after you” – is the latest sign of the Tories seeking to burnish their credentials as the party of law and order ahead of an expected election.
As the countdown to the Brexit deadline of October 31 continued, the PM insisted the UK would quit the EU on that date “whatever happens”.
Meanwhile, The Times reported that Mr Johnson’s plan to get around the Benn Act – the law aimed at preventing a no-deal Brexit without MPs’ approval – would be to ask EU leaders to rule out any extension to the October 31 deadline.
He would then seek to present MPs in Parliament with a straight choice of agreeing the revised deal or leaving without an agreement on Halloween.
Conservative Party chairman James Cleverly heavily hinted the Government believed there was a way of wriggling out of the Benn Act.
He told a Politico fringe event in Manchester on Monday evening: “Legislation passed in a rush tends to be bad legislation.”
He refused to say how the Government could get around the legislation, adding: “What we’ve seen is parties distorting the parliamentary process, breaking conventions, taking a very creative interpretation of parliamentary procedures to prevent the Government discharging a promise the Prime Minister made and indeed a promise that all parties made at the referendum.
“And I’m not going to help them by showing them our homework.”