Video report by ITV News Political Correspondent Carl Dinnen
Mr Johnson urged people to stay two metres apart and said social distancing was "absolutely critical."
Speaking at the daily press conference at Downing Street, the prime minister did not announce any new measures, but said he could not rule out having to enforce tougher rules if people ignored the current advice.
His warning came as the UK death toll from Covid-19 has risen by 48 to 281 - including an 18-year-old with underlying health conditions, thought to be the youngest victim so far.
A total of 5,683 patients have tested positive for coronavirus in the UK.
Mr Johnson said he wanted people to continue to "enjoy themselves" amid the pandemic, but added it was "absolutely crucial" that people followed guidelines.
"I want, of course I do, people to be able to go to the parks, open spaces and enjoy themselves," he said.
"It is crucial for health, physical and mental well-being."
"But please follow that advice and don't think that fresh air in itself automatically provides some immunity.
"Take this advice seriously, follow it, because it's absolutely crucial."
The prime minister's latest comments come as some parks, beaches and beauty spots were overwhelmed with visitors over the sunny weekend.
Snowdonia National Park Authority (SNPA) called on the government to act after the area saw "unprecedented scenes" on Saturday which saw hundreds of people walking up Wales' highest mountain in what the park say was "the busiest visitor day in living memory".
A police commissioner called for “lockdowns” in rural Wales to stop self-isolationists and tourists flooding in.
Dafydd Llewelyn, who is the Plaid Cymru police and crime commissioner for Dyfed Powys, described the Covid-19 epidemic as a “national emergency, not a national holiday”.
He has called for travel and residency restrictions to be introduced, and people travelling to a non-primary residence should be stopped from doing so while those currently in non-primary residences should return home.
The Government said people should avoid travelling unless it is essential, and urged the public not to visit holiday or second homes, "whether for isolation purposes or holidays."
Mr Johnson said tougher measures would be introduced if people failed to heed warnings about social distancing.
He said: "Going outside now and taking exercise - you've got to take account of the medical advice and observe social distancing.
"If people can't do that, won't do that, don't do that then yes of course we're going to have to bring in tougher measures."
He added: “I don’t think you need to use your imagination very much to see where we might have to go, and we will think about this very very actively in the next 24 hours.”
Mr Johnson said the Government had taken "very draconian steps" in closing schools, pubs and shops.
He said: "A huge quantity of our normal daily life has been totally transformed."
Other political leaders adopted a notably more strident tone than the prime minister as they warned people not to leave their homes other than for essential business.
What is social distancing?
London Mayor Sadiq Khan, whose city is at the centre of the epidemic in Britain, said people must obey the guidance or others would die.
“This isn’t advice, as far as I’m concerned. These are instructions and these are rules that we should all obey to stop people dying,” he told BBC1’s The Andrew Marr Show.
Scotland’s First Minister Nicola Sturgeon announced she was closing the ferries to the Scottish islands to all non-essential travellers after the military was brought in to transport a patient to the mainland.
She said people should not go shopping “except for essentials like food and medicine” and she condemned people “flocking” to Scotland’s remote communities for adding to the pressure on local health services.
“Beaches should not be busy, parks should not be full,” she said.
Meanwhile the NHS is to begin sending out letters to the 1.5 million people considered to be most at risk of the disease urging them to remain at home for the next 12 weeks.
Mr Johnson said the “shielding” of the 1.5 million people considered to be the most vulnerable to coronavirus would do “more than any other single measure to save life”.
They include people with severe respiratory conditions and those suffering from some cancers such as those of the blood or marrow.
Communities Secretary Robert Jenrick said that, with the help of military planners, the Government was creating a network of local “hubs” to ensure those without family or friends to support them received their medicines and other vital supplies.
“Nobody needs to worry about getting the food and essential items that they will need,” he said.
Coronavirus: Everything you need to know